Competent and quick setup of the mail server. Mail Email mail servers

Mail server (email server, mail server)- in an email forwarding system, this is usually called a message transfer agent (MTA). This computer program, which transmits messages from one computer to another. Typically, the mail server works “behind the scenes”, and users deal with another program - an email client (mail user agent, MUA).

Organizing your own mail server allows you to create a more flexible policy for sending and receiving mail messages. Often, the capabilities of a mail domain hoster to configure and process letters are limited, while ICS filters allow you to simulate many different situations, as well as keep records and statistics of transmitted messages, and much more.

When entering the module, the status of all mail and jabber server services is displayed, along with the “Disable” button (or “Enable” if the service is disabled). There is also a widget with a selection of main actions, a graph of mail statistics and mailer feeds, as well as the latest log events.

Settings

The Settings tab allows you to define following parameters mail server:

SMTP/POP3/IMAP port- allows you to change the standard ports for receiving and sending mail messages.

Interfaces for SMTP/POP3/IMAP- allows you to select server interfaces through which mail messages are received and sent. By default, all interfaces are enabled.

The following fields allow you to set the maximum letter size (in megabytes), the maximum number of letters from one IP address per minute, the interval between sending attempts, as well as the minimum and maximum waiting time in the mail queue.

Rayleigh by default. Relay is a node that is engaged in receiving/forwarding messages (e-mail), in this case its default role is played by the ICS. In some cases, you may need to register another server through which the ICS will send mail (for example, in the case of a multi-drop mailbox configured on the provider's mail server).

Addresses from which forwarding is allowed- this is a list of addresses and domain names from which the ICS will always accept mail without checking with gray lists and checking the correspondence of forward and reverse records.

Addresses from which forwarding is prohibited- this is a list of addresses and domain names from which the ICS will always reject email messages.

RBL blacklists. RBL, Real-time Blackhole List (or DNSBL - DNS blacklist or DNS blocklist) - lists of hosts stored using the DNS architecture system. Typically used to combat spam. The mail server accesses the DNSBL and checks it for the IP address of the client from which it is receiving the message. If the answer is positive, it is considered that an attempt is being made to receive a spam message. The sending server receives a 5xx error (fatal error) and the message is not accepted. In most cases, you do not need to change this list.

Default domain for authorization defines the email domain that will be automatically substituted when the user logs in. If you specify a default domain, users of this domain will be able to log in by mailbox name without specifying a domain.

When creating a mailbox, automatically create folders- contains a list standard folders created in the mailbox. If necessary, you can change their composition.

Check email with Clamav / Dr.Wed / Kaspersky antivirus- checking these flags signals the mail server to check incoming and outgoing messages for viruses. If the result is positive, instead of the letter itself, the recipient will receive a message about the results of the check, and the letter itself will be attached to the message.

Use greylisting. Greylisting is a method of automatically blocking spam, based on the fact that the “behavior” of software designed to send spam differs from the behavior of regular email servers. If the recipient's mail server refuses to accept the message and reports a “temporary error,” the sender's server must try again later. Spammer software in such cases, usually, does not try to do this. You can enable this mode to enhance email scanning for spam. After enabling this option, gray list parameters will become available for editing - time to ignore re-send (in seconds), time to wait for re-send (in hours), time for keeping the sender in the white list (in days).

Server name for SMTP defines the SMTP Banner Postfix parameter.

Certificate for SMTP/POP3/IMAP- like other ICS services, the mail server can operate either using a standard protocol, in which data is not encrypted when transferring files (which is unsafe), or using a secure one. SSL certificates are used for these purposes. By clicking on the […] button, you can assign a certificate pre-created in the corresponding module to each protocol.

Use DLP- launches a service for checking email messages using fingerprints of confidential information.

Hard drive for mail storage allows you to move your mail storage to a separate one HDD. By default, mail is stored on the system partition.

Signature for the web interface, it is enabled in the mail server settings; to do this, you need to check the “use signature” checkbox and enter the signature in the window that opens by clicking the “edit html” button, and then save the settings.

The signature can be entered in both wysiwyg and html mode.

In the signature you can use variables in the form [variable name], their possible values ​​are indicated below:

Cn - Username ou - Group in which he is located mail - Mailing address description - Field "description" of the user notes - Field "comment" of the user telephonenumber - Field "telephone" of the user title - Field "position" of the user url - Field "Website" of the user postaladdress - Field "Address" of the user pager - Field "ICQ" user ounotes - Field "description" of the group in which he is located

To insert images, the image is encoded in data:url. This is done as follows: using the service http://dataurl.net/#dataurlmaker (or similar), the image is converted into , then the resulting text is inserted into the signature html code.

IMPORTANT: the peculiarity of this mechanism in Roundcube is that the signature will be automatically generated only for a new account created after setting up the signature. After generation, it will also not be possible to change the signature automatically, so carefully plan the automatic signature when creating

Upload logo for Roundcube- this button allows you to select an image that will be located in the upper left corner of the mail web interface. For example, your organization's logo.

The last three checkboxes allow you to enable DKIM signing, check the DKIM of incoming messages, and automatically recode message headers to UTF-8.

The address book

Domains and boxes

Before you can add custom mailboxes, you must create a mail domain. Go to the “Domains and Mailboxes” tab and click the “Add” → “mail domain” button. You can call the domain any non-existent name if the exchange of letters will take place inside corporate network, or set up message forwarding on a real-life domain registered with your organization. If the “Create DKIM signature” checkbox is selected in the settings, it will be added automatically.

When you double-click on the created account again, it will open with the already created DKIM key, which can be copied if necessary.

After that, by selecting the created domain, you can add user mailboxes to it. The server will ask you to enter the mailbox name, password and select the user behind which this box will be fixed. If necessary, you can specify a quota - the maximum reserved space on the IKS hard drive for storing letters given user. Once this quota is exceeded, no emails will be accepted for the user. By default, there is no quota.

It is not necessary to create a separate Mailbox for every mailing name you need. Instead, you can create a link to the specified mailbox. Then all the letters coming to the mailbox [email protected], will be redirected to a real mailbox [email protected].

Important: when creating mail domains and mailboxes, the corresponding domains and accounts appear in the jabber server section. The opposite is also true.

In order for the mail domain to be accessible from the external network and be able to exchange data with others external servers, you need to configure DNS records.

After mailboxes for users are created, they can connect to the IKS using email clients (for example, Mozilla Thunderbird or Microsoft Outlook) or use web interface for mail.

To process sent and received letters, use the “Filters” tab. They process mail based on the following conditions: size, sender, recipient, subject. The condition can be strict or non-strict. The number of conditions can be any, and the filter can process mail both when all conditions are completely matched, and when there is the first match. After the condition is matched, the filter can delete the letter, move it to another mailbox, or make a copy.

In the above example, all letters arriving at one address, the size of which is more than 5000 kB, and the subject of the letter contains the expression “not spam” will be copied to another ICS mailbox.

In order to create a new filter, you first need to select trigger conditions - if all conditions match, any of the conditions, or apply to all messages regardless of the conditions.

You can filter incoming and outgoing letters by subject, sender, recipient and size (in kilobytes). The check for matching a condition can be strict (“matches with”) or non-strict (“contains,” “starts with,” “ends with”), as well as the reverse (“does not contain”). You can assign any number of conditions to one filter.

The last step is to select the action that occurs after the filter is triggered. You can move the letter, copy it to another address, or delete it. The first two conditions allow you to enter the name of the mailbox or select it from the list of those created on the ICS.

For an example of how filters can be configured, see.

Mailings are the same filters, but with a simplified interface in which it is enough to indicate the mailboxes to which the mailing will be distributed. The mailbox to which the original letter arrives should not be opened in the system, since it is a link.

Antispam

To manage mail accounts located on other servers, you can use the ICS “mail collector” function. With its help, the ICS connects to the specified mail server under the selected login and password and moves or copies the contained mail to the mailboxes of ICS users.

You can specify what to do with messages on the server - collect all, collect only new ones, leave messages on the server, or delete them. You can also configure the collector's operating interval and the number of downloaded emails per session.

It works in two modes - automatic recipient detection and specifying a mailbox for collection. Automatic detection works if the organization has one external mailbox located on the provider’s server, and the remaining mailboxes serve as its aliases. In other cases, a direct indication of the assembly box is used. That is, in most cases, when creating a collector, you need to set the switch to the “Forward to” position.

The mail collector can also be used in cases where the organization uses the so-called "multi-drop" mail method. It consists in the fact that all mail arrives at the server of the provider or hoster and is stored there without being divided into user mailboxes. In this case, when setting up the mail collector, the “recipient” field does not need to be changed (the default value in it is the recipient’s address). Thus, the collected letters will be automatically distributed depending on the addressee among the mailboxes of ICS users, and if there are no such addressees, they will be stored in the mailbox selected by default.

Postal queue

This tab shows messages that are waiting to be sent, or that for some reason were not sent (for example, rejected by the greylist of an upstream mail server). When you select any object from the list, you can see the error code for which it was not delivered. You can manage the mail queue using the “Clear Queue” and “Send All” buttons. Also, you can try to send each letter individually or remove it from the queue.

Statistics

To control incoming and outgoing mail traffic, as well as spam and unwanted letters, you can use the “Statistics” section.

Just like in user statistics, you can apply various filters in the control panel to general information about ICS mail traffic and display them in the form of a table. The table columns vary depending on the filter applied.

The report generator looks a lot like custom statistics. Basic filters can display information about user traffic, grouping them according to the following criteria:

by sender domains,

by recipient domains,

by mailboxes,

by hour/day/month,

details of letters;

Magazine

The “Log” tab contains a summary of all system messages from the mail server. The magazine is divided into pages; using the “forward” and “back” buttons you can move from page to page, or enter the page number in the field and switch directly to it.

Log entries are highlighted in color depending on the type of message. Regular system messages are marked in white, errors are marked in red.

There is a search bar in the upper right corner of the module. With its help, you can search the journal for the entries you need.

The log always displays events for the current date. To view events on a different day, select the desired date using the calendar in the upper left corner of the module.

Many system administrators experience certain difficulties when working with email systems. This is not surprising; a mail server has a much more complex structure than a file server, router or terminal server. In this article we will look at the structure and operating principle of mail servers, without understanding which setting up an email system can easily turn into shamanic dances with a tambourine.

This material contains quite a lot of simplifications and generalizations in order to give system administrators required minimum knowledge. In our opinion, for the sake of administering one or two mail servers entry level You don't have to become an email expert.

For most users and novice administrators, a mail server is a kind of “black box” that, having received a letter through “unknown” ways, delivers it to the recipient and vice versa. All interaction with such a server consists of the mail client contacting certain ports, or even through the web interface. However, there is a whole mechanism hidden inside, understanding the operation of which is key to successfully setting up and maintaining an email system. This is especially important for administering servers on the Linux platform. Unlike Windows, where the mail server is a complete software solution and the developers have already taken care of internal interaction, in Linux the components of the mail server are separate programs and you need to configure their interaction yourself.

Let's look at the structure of the mail server, as well as what happens when a user tries to send mail.

The most important part of the mail server is MTA (Mail Transfer Agent-- mail forwarding agent) whose tasks include receiving and transmitting mail. Very often (in Linux / UNIX) the MTA is also called a mail server. MTA operates using the SMTP protocol, and it alone is, in principle, sufficient to create an email system. Once upon a time, this was exactly the case, and to access your mailbox you needed to have certain technical knowledge.

However, progress does not stand still; the MTA, upon receiving a letter, places it in the user's mailbox on the server, which the latter must access, preferably in the simplest and most understandable way. This is where it comes into play MDA (Mail Delivery Agent-- mail delivery agent), its task, at the request of an email client, is to transfer mail from a mailbox on the server to it. MDA can operate using the POP3 or IMAP protocols; in some cases, proprietary protocols with advanced functionality, such as MAPI (Exchange Server), can be used to “communicate” between the mail client and the delivery agent.

Contrary to popular belief, MDA has nothing to do with the mail transfer process. That's the MTA's prerogative. To make an analogy, MTA can be thought of as Postal office, which is responsible for receiving and sending mail, and MDA with the postman, who brings the mail to your home. If the postman gets sick, this will not affect the work of the post office in any way, you just won’t receive letters at home. Also MDA, its failure does not lead to the inoperability of the mail server, only receiving mail by the mail client becomes unavailable, at the same time it can be easily accessed in other ways, for example, through the web interface.

Let's see what happens when sending mail. In our example, the user Ivanov, located in the example.org domain ( [email protected]), writes a letter to Kozlov in the example.com domain ( [email protected]). For Ivanov, the process of sending mail consists of creating a message and clicking the "Send" button in the email client. The mail client connects to the MTA using the SMTP protocol and first provides its credentials. After authorizing the user, the MTA accepts the message and tries to deliver it further.

In fact, authorization is not a mandatory procedure for MTA, but without authorization we will get an open relay, i.e. Anyone can use our server to forward mail, and how spammers will rejoice! Currently, open relays occur mainly due to server configuration errors. However, it is quite possible for an MTA to accept mail from trusted users without authorization, for example from local network enterprises.

For authorization, the MTA can use its own user list, system list, LDAP or AD user lists. There is also a way: POP authorization before SMTP, when the user authorizes the MDA before sending mail, which in turn confirms the user's authentication to the MTA.

The next step is for the MTA to analyze official information letters, determining the recipient's domain, if it belongs to the domains served by this MTA, the recipient is searched and the letter is placed in his mailbox. This would have happened if Ivanov had written a letter to Petrov or Sidorov.

If the recipient's domain is not served by an MTA, a DNS query is generated asking for MX records for that domain. An MX record is a special type of DNS record that contains the names of the mail servers that process incoming mail for a given domain. There can be several MX records; in this case, the MTA tries to establish a connection sequentially, starting with the server with the highest priority. If there is no MX record, an A record is requested (the address record that matches Domain name with an IP address) and an attempt is made to deliver mail to the host specified there. If it is impossible to send a message, it is returned to the sender (placed in the user's mailbox) with an error message.

We will not consider the work of the receiving server, we will assume that everything went fine, Kozlov received a letter from Ivanov and wrote him a response. The server serving the example.com domain performs exactly the same actions and tries to transfer mail to our server. Having received incoming message The MTA, as in the case of a local sender, checks the recipient's domain, if it is among those served by the MTA, message processing continues, otherwise the server refuses to accept mail. After checking the domain, the recipient is checked, if he is present in the list of users, the message is delivered to his mailbox, otherwise two options are possible: refusing to receive the message or receiving the message in the general mailbox (administrator's mailbox). On the one hand, this setting increases the number of spam received, on the other hand, it allows you to avoid losing letters with misspelled addresses.

Another anti-spam measure is to request a PTR record. A PTR record (pointer record) associates an IP address with a domain name. By requesting PTR, the MTA only accepts mail if the sender's domain matches the domain of the sending server.

Let's look at the example in more detail. A certain spam server spam.com is trying to send letters with a fake sender, supposedly from the example.com server we know. If filtered by white/black lists, such a letter will be delivered, since the sender is a user from a trusted domain (which is what spammers were counting on). In order to combat spam, the MTA generates a PTR record request for the IP address of the sending server, which it reports during the SMTP session. For the address y.y.y.y, the PTR request will return the spam.com domain name, which does not match the sender’s domain, which will cause the message to be rejected. At the same time, messages from the server x.x.x.x will be received, since the domain from the PTR record for x.x.x.x (example.com) matches the sender's domain.

So, the message has been received and is in the user's mailbox. How to read it? Mail storage, where user mailboxes are located, can be organized in the most different ways: starting from banal folders and files, ending with a database. Without technical knowledge, it is unlikely that you will be able to read your own mail. But should user Ivanov really care about this? For him, the process of receiving mail is reduced to clicking the "Receive" button in the mail client.

To receive mail, the client establishes a connection with the MDA using the POP3 or IMAP protocol, always passing authorization data. MDA checks the presence of the user in the lists and, if the check is successful, transfers to the client all new messages in his mailbox. User Ivanov receives his correspondence and can work with it in a way convenient for him.

This is where our article ends; we strongly recommend thoughtful reading and assimilation of the material presented in it. Subsequently, when considering practical implementations of mail servers, we will present the material on the assumption that the reader has knowledge of at least this article.

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When launching a website or web application, users typically configure a mail server to process incoming and outgoing mail. However, often having your own mail server is a serious vulnerability for a project for a number of reasons. In general, setting up and maintaining a mail server is a complex, time-consuming and time-consuming task.

This guide will cover many of the reasons that might keep you from using an email server, and will also help you find alternative options.

Difficulties of mail servers

A typical mail server consists of many software components, each of which performs a specific function. Each component must not only be fine-tuned, but also interact with other components to ensure full functionality of the mail server. Because of large quantity The “moving” components of mail servers are quite difficult to install.

Components required by every web server:

• Message transfer agent (MTA);
• Message delivery agent (MDA);
• IMAP and/or POP3 server.

• Spam filters;
• Antiviruses;
• Webmail (webmail).

Some software packages include functionality several components at once. In addition to software components, mail servers require a domain name, corresponding DNS records and SSL certificate.

Let's look at the necessary components in more detail.

Message Forwarding Agent

The message transfer agent (or MTA), which processes SMTP (Simple Mail Transfer Protocol) traffic, is responsible for:

• sending user messages to an external MTA (that is, to another mail server);
• receiving messages from an external MTA.

Examples of MTA programs: Postfix, Exim, Sendmail.

Message delivery agent

The Message Delivery Agent (or MDA), sometimes called the Local Delivery Agent (LDA), retrieves messages from the MTA and places them in the appropriate user's mailbox.

There are a huge variety of mailbox formats (for example, mbox and Maildir). Each MDA supports a specific mailbox format. The mailbox format determines how messages are stored on the mail server, which in turn affects disk usage and the speed at which the mailbox is accessed.

Examples of MDA programs: Postfix and Dovecot.

IMAP and/or POP3 servers

IMAP and POP3 are protocols used by email clients, that is, any software for reading and retrieving messages. Each protocol has its own challenges; This article looks at some of their key differences.

IMAP is a more complex protocol that allows, among other things, to support multiple clients to connect to an individual mailbox at the same time. Email messages are copied to the clients, but the original remains on the mail server.

The simpler POP3 protocol moves email messages to your email client computer (usually the default local computer user).

Examples of IMAP and POP3 servers: Courier, Dovecot, Zimbra.

Spam filters

The purpose of a spam filter is to reduce the amount of incoming spam or junk mail, ending up in users' mailboxes. To achieve this goal, spam filters apply spam detection rules that take into account various factors (message sender, content, and more) to evaluate the message. If this score exceeds the so-called spam level, the message is recognized as spam.

Spam filters can also be applied to outgoing mail. This function may be useful if mail account user has been hacked: this way you can reduce the amount of spam transmitted using this mail server.

Popular filter with open source code is .

Antiviruses

Antiviruses are used to detect viruses, Trojans, malware and other threats in incoming and outgoing mail. ClamAV is a popular open source antivirus.

Webmail (or Webmail)

Many users need webmail. Webmail (in the context of working with a mail server) is mail client, accessible to users via a web browser (Gmail is probably the most famous example). This component requires a web server (eg Nginx, Apache) and can run directly on the mail server.

Examples: Roundcube and Citadel.

Mail server support

Now that we've become familiar with the email server components that need to be installed and configured, let's consider the following question: Why can maintaining an email server become too labor-intensive and time-consuming? There are routine tasks Maintenance: updating antivirus and spam filter rules, keeping all components up to date, etc.; but besides this there are many other points.

Blacklists

One common problem with maintaining a mail server is the need to keep it off blacklists (also called DNSBL, blacklists, or blackhole lists). Such lists contain the IP addresses of mail servers that distribute spam or junk mail, as well as servers with incorrectly configured DNS records. Many mail servers subscribe to one or even more of these blacklists and then filter incoming messages based on whether the mail server that sent the message is on the list (or lists). If a mail server goes into an emergency, its outgoing messages will be filtered (and some blocked) before being sent to the recipient.

As a rule, a mail server involved in an emergency can be excluded from this list. To do this, you need to find out the reason for blacklisting the server and eliminate it. Then you need to follow the process of removing the server from the lists and subscribe to at least one of them.

Troubleshooting

Although most people use email every day, not everyone can troubleshoot it. complex system. For example, what should you do if sent messages were not accepted by the recipient? The problem may arise due to incorrect configuration of one of the many components of the mail server (for example, due to a poorly configured outgoing message spam filter, or external factors such as blacklists).

Today, when you can easily get any number of free mailboxes by simply registering an account on the services of Yandex, Mail.Ru and the like, such an issue as setting up a mail server does not bother too many users.

However, there are also times when knowing at least some aspects of this issue will be useful.

When it is needed

There are not so many situations when basic knowledge of setting up mail is needed by the average user. And yet, any of them can happen at any time:

• If it was decided to switch from browser-based to dedicated in a separate program. Indeed, in many respects such clients are preferable: they allow you to combine mail program with organizer, notebook, as well as more flexible management of your address book and message administration.
• An unexpected failure occurred in the email client and all settings were lost. And then you just need to install a mail server. Setting it up usually does not require much time and effort, but otherwise you can be left without mail for quite a long time.
• Free mailboxes can be destroyed by the administration, without explanation. And, frankly speaking, such a box looks undignified in the eyes of business partners. Therefore, you will have to create a dedicated one on the server.
• If your provider offers a separate mailbox, then why not take advantage of this offer.

Setting up a Windows mail server

Basic email parameters, such as DNS, IP data and similar information, are issued directly by the provider.

To start using Windows OS, you will need to either download the appropriate one for this operating system client, or use the built-in clients. First you need to create a new one account. As a rule, they are also asked to enter its name, come up with a password and login to log in.

You may need to enable email support Windows services through the "Uninstall or Install Programs" panel in the E-mail Services components installation section.

To create a new mailbox, you will need to come up with a username and password.

In SMTP you must specify the port number 25, and for the POP3 server - 110. If the provider provides other parameters, you must enter them. In the case when the mail client you are using does not require entering a port number, you need to leave only the address issued by the provider in the “Server for incoming messages” (this can be either POP3 or IMAP) and “Name for the server for outgoing messages” ( usually SMTP only).

More fine tuning postal Windows server largely depends on what is used mail application, but the operating principle will be the same. The difference may lie in the options GUI and in menu items.

Switching from free email to a dedicated client

Sometimes you need to stay on free but use it as a client separate application. You can show this using the example of mail settings for the Yandex service. The mail server will then be configured with the following parameters.

1. Settings by IMAP protocol for incoming messages:

• mail server address: imap.yandex.ru;
• SSL should be specified in the security settings for the connection;
• The port number is 993.

2. To configure outgoing messages via IMAP:

• specify smtp.yandex.ru as the server address;
• You must also set SSL in the connection security parameters;
• The port number should be set to 465.

3. Regarding the POP3 protocol for sent messages:

• specify pop.yandex.ru as the server address;
• SSL is specified as the security parameters of the connection used;
• The port number is 995.

4. For outgoing messages sent via POP3:

• smtp.yandex.ru is indicated as the mail server address;
• in the security parameters for the connection used, SSL is again indicated;
• The port number is set to 465.

As a username, as well as an address and password, you should set already existing addresses and passwords from mail on Yandex.

Setting up the Mail.Ru server

Sometimes you need to find out about the settings of the Mail.Ru mail server. In general, the setup looks exactly the same as described in the case of Yandex mail. But the parameters will look like this:

• full email address(in format with @ sign, for example [email protected]);
• for the IMAP server, imap.mail.ru is specified;
• for the SMTP server, specify smtp.mail.ru;
• the username is the full email address from an existing mail;
• password is the mail password used;
• IMAP: number 993 (for SSL/TLS protocols);
• POP3: number 995 (for SSL/TLS protocols);
• SMTP: number 465 (for SSL/TLS protocols);
• you need to indicate that you want to authorize the server of sent letters, in the authentication parameters - a simple password, without encryption.

In general, the settings are exactly the same as in the case of Yandex, but only with the addition of the mail prefix. For other free servers, you should set the same parameters, but with the appropriate prefixes.

As you can see, in general there is nothing complicated in such a matter as setting up a mail server. Even a novice user can cope with this task. But you can be sure that even in the event of a critical failure you will not be left without mail.

The task of setting up your own mail servers using Linux tools, Apache, SQL and the like requires deeper knowledge in the field of information technology.

Introduction

Even in ancient times, people felt the need to exchange information and organized it at the state level. Therefore, the post office is one of the most clearly organized institutions in the world.

Email is new modern means transfer of information. Unlike regular mail, email is used to transmit electronic copies messages, files, programs, various data - i.e. information processed using a computer.

The main objects that make up an email system are special computers, called mail servers.

Mail server

Mail servers are servers that receive and send emails.

The server that receives emails uses POP (Post Office Protocol).

The server sending emails uses the SMTP (Simple Mail Transfer Protocol) protocol.

Mail server, email server, mail server - in an email transfer system this is usually called a message transfer agent (MTA). It is a computer program that transmits messages from one computer to another. Typically, the mail server works “behind the scenes”, and users deal with another program - the email client (mail user agent, MUA).

Rice. 1.

For example, in a common configuration, the user agent is Outlook Express. When the user types a message and sends it to the recipient, the mail client communicates with the mail server using the SMTP protocol. The sender's mail server interacts with the recipient's mail server (directly or through an intermediate server - relay). On the recipient's mail server, the message enters the mailbox, from where it is delivered to the recipient's client using a message delivery agent (MDA). Often the last two agents are combined in one program, although there are specialized MDAs that also deal with spam filtering. For final delivery of received messages, a protocol other than SMTP is used - often POP3 or IMAP - which is also supported by most mail servers. Although in the simplest implementation of MTA it is enough to put received messages in the user’s personal directory in file system central server (“mailbox”).