Mail servers and their capabilities. How does email work? Self-configuration manual

Introduction

Back in ancient times, people experienced the need for the exchange of information and organized it at the state level. Therefore, the post office is one of the most well-organized institutions in the world.

Email is a new modern means of transmitting information. Unlike regular mail, e-mails are transmitted e-mail, files, programs, various data - i.e. Information processed using a computer.

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

Mail server

Mail servers are servers receiving and sending emails.

The server receiving e-mails works via POP (Post Office Protocol).

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

Mail Server, Email Server, Mail Server - In the Email Transfer System, this is usually called message forwarding agent (eng. Mail Transfer Agent, MTA). This is a computer program that transmits messages from one computer to another. Usually, the mail server works "behind the scenes", and users deal with another program - an email client (eng. Mail User Agent, MUA).

Fig. one.

For example, in a common configuration, the user agent is Outlook Express. When the user scored a message and sends it to the recipient, the email client interacts with the mail server using the SMTP protocol. The sender's mail server interacts with the recipient's email server (directly or through the intermediate server - relay). On the mail server of the recipient, the message falls into the mailbox, from where using the mail delivery agent (Mail Delivery Agent, MDA) is delivered to the recipient's client. Often the last two agents are combined in one program, although there are specialized MDAs, which, among other things, are engaged in spam filtering. For the final delivery of received messages, not SMTP is used, and another protocol is often POP3 or IMAP - which is also supported by most mail servers. Although in the simplest implementation of MTA, it suffices to put the received messages to a personal user directory in the center system of the central server ("Mailbox").

South Ural State University

Mias Engineering Faculty

Department of Quality Management and Standardization

Course work

By computer science

On the topic "Mail Server"

Introduction _________________________________________________________3

Mailing server _________________________________________________________4

Email ____________________________________________________________

Email Address Structure ____________________________________________

What is domain __________________________________________________________7

The story of the sign @______________________________________________________________ 9

Via email ___________________________________________________11

Mail.ru __________________________________________________________________ 12.

Yahoo! Mail _____________________________________________________________15

Rambler.ru ______________________________________________________________17

Leading postal clients _______________________________________________17

Conclusion _____________________________________________________________18

References ___________________________________________________________ 19

Introduction

Email is a new modern means of transmitting information. Unlike regular mail, e-mails are transmitted e-mail, files, programs, various data - i.e. Information processed using a computer.

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

Mail server

Mail servers are servers receiving and sending emails.

The server receiving e-mails works via POP (Post Office Protocol).

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

Mail Server, Email Server, Mail Server - In the email forwarding system, this is usually called the sending agent (eng. mail TRANSFER Agent, MTA). This is a computer program that transmits messages from one computer to another. Usually, the mail server works "behind the scenes", and users deal with another program - an email client (eng. mail User Agent, MUA).

Scheme of interaction

Email

Email (e-mail, from Latin "Electronic Mail").

Email, as well as normal, works with the electronic "post office" system - mail servers that provide sending letters on global networks. They interact using postal protocols that provide shipment and recognition of information transmitted to the network. Mail Server Client Computers serve email users. Each receives its email address and its "mailbox" on this computer, i.e. Memory area, as well as password to access it.

Using the mail program, you can create messages, read them from the mail server, to work with the address book, store and organize letters in the mailbox folders, prepare files for shipment and convert them to the desired format after receiving, etc.

Using the mail program, the user creates a message to the addressee, sets the address, sends a message for which it is connected to the mail server. During the connection, the mail server requests the username and his password. Otherwise, the communication session will not take place. After the connection, the prepared mail is automatically sent to the server and further through the transmission from one to another postage server reaches the addressee. Immediately after sending correspondence, the mail is automatically received by the client. Messages read in its area and files are sorted and folded through user mailboxes. The addressee when loading your box sees the post-folder posts: new, old, sent. It can delete, sort and classify them in its understanding.

Email Address Structure

When sending information, the addressing is of great importance, since the recipient cannot be found without it. Everyone knows the sad story of Vanka Zhukov, who sent a letter "on the village of Grandfather." The usual mail address is issued according to a specific postal rule.

Existing rules for designing email addresses other. Email addresses have a clearer logical structure. They consist of a hierarchical sequence of domains - parts, for example:

[Email Protected]

All addresses consist of two parts separated by @ symbol (read "ET"). By reading from left to right to this sign, users are displayed (recipients). This can be the name of the head of the post office - "Post Master" (Post Master), fictional or true names of users of e-mails that correspondence comes. They can be registered on the same computer a lot. Part of the address on the right of @, defines a computer connected to the network, city and country or network name in which the user is registered. Addresses are divided into parts called domains.

What is domain

Considering the domain to the right left and smashing it at the points for individual words, we get subdomains, alternately clarify, where this mailbox is search. Analogy with ordinary mail domain is the address (line "where" on the envelope), and subdomains are the name of the country, cities, streets, house number.

Domain does not describe the path for which the message should be transmitted, but only explains where the addressee is located; Similarly, the address on the mail envelope is not a description of the road at which the postman should go to deliver the letter, and the place in which he must eventually bring him. In both cases, postal services themselves choose the route for reasons of time saving and money. Usually there are several ways for which you can deliver a message to the specified location, and, sending a letter, you do not know how it will go from this time.

The right right subdomain (in our case, RU) is called the top-level domain and most often denotes the country code in which the server is located. The RU code is Russia, KZ - Kazakhstan. Each code consists of two Latin letters. For example, the UK code refers to the UK, and the mailbox with the address [Email Protected] It should be sought in the English Janet network.

The top level domain is not always the country code. In the United States there are such, for example, top-level domains, such as EDU - scientific and educational organizations, or GOV - government agencies:

[Email Protected] rge.arc.nasa.gov.

If the postal service sees the subdomain of this species on the right side of the domain, it already knows that the destination is located in the United States, so the country code is not needed. Such designations have developed in the American scientific network of Arpanet before it was associated with networks in other countries, and now they are saved only by habit. As a rule, in all places that are addressed by the type of organization, you can get and using the country code. For considerations of simplicity and uniformity, it is better to use addresses with codes of countries.

Typically, such addresses are used if this network understands the addresses in a format different from RFC822. Then you write the type of name@Mashin. Sails, and the bridge between your network and the destination network converts it to the right mind.

Subdomains located the right of the top level domain, refine the position of the addressee within this domain (within Russia for RU, among US military organizations for MIL, or on the BITNET network for BITNET). For example, in the address [Email Protected] The DEMOS subdomain means an organization within Russia, and HQ is a group of machines inside Demos.

In the addresses [Email Protected] The top-level domain Gov means that the addressee is in one of the US government agencies, the first NASA subtrerage specifies, in which NASA, the second subdomain of the ARC calls the NASA division - Ames Research Center, and George points to a specific machine in this unit.

If the letter is addressed by the name of the network, in which it must be sent, the address (domain) consists only of the top level domain - the network name and one more subdomain - the name of the car in this network. To understand where this machine is located, the email services of this network falls.

When you need to reach an address, for example, uX.. cSO.. uIUC. eDUThe computer must convert it to the address. To do this, your computer begins to ask for help from servers (computers) DNS, starting with the right side of the name and moving to the left. At first she asks Local DNS servers to find the address. There are three possibilities here.

Mail Server, Mail Server, Email Server - is usually called a message forwarding agent in the email shipment system. This software that transmits messages from one computer to another. The mail server is usually not visible for the user. The user itself deals with another email client.

For example, in the most common configuration, the mail client is Outlook Express. Although the Mozilla - Thunderbird is often used in recent times. When a user message dialing and sending it to the recipient, the email client interacts with the mail server using the SMTP protocol. The postgraduler's postal server interacts with the recipient server. On the recipient server, the message sent to him falls into the mailbox, from which, using the MDA message delivery agent (Mail Delivery Agent), is delivered to the recipient client. There are also POP3 and IMAP protocols that are supported by many mail servers.

, | |

Running site or web application, users tend to configure both the mail server for the processing of incoming and outgoing mail. However, often the presence of its own mail server is a serious vulnerability for the project according to a number of reasons. In general, the configuration and support of the mail server is a complex and time-consuming task that takes a lot of time.

This manual covers many reasons that can keep you from using the mail server, and will also help you choose alternative options.

Complexity of mail servers

An ordinary mail server consists of a variety of software components, each of which performs a specific function. Each component should not only have a fine setting, but also interact with the rest of the components to provide the full functionality of the mail server. Due to the large number of "movable" components, mail servers are quite difficult to install.

Components needed to each web server:

Message forwarding agent (MAIL TRANSFER AGENT, MTA);
Message Agent (Mail Delivery Agent, MDA);
IMAP server and / or POP3.

Spam filters;
Antiviruses;
Webmail.

Some software packages include the functionality of several components immediately. In addition to software components, mail servers require a domain name corresponding to DNS entries and a SSL certificate.

Consider the necessary components in more detail.

Agent sending messages

Message forwarding agents (or MTA), machining SMTP protocol traffic (Simple Mail Transfer Protocol), is responsible for:

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

Examples of MTA: PostFix, Exim, Sendmail.

Message delivery agent

The message delivery agent (or MDA), which is sometimes referred to as a local delivery agent (Local Delivery Agent, or LDA), extracts messages from the MTA and places them in the mailbox of the appropriate user.

There are a huge set of mailbox formats (for example, MBox and Maildir). Each MDA supports a specific mailbox format. The mailbox format defines a method for storing messages on the mail server, which, in turn, affects the use of the disk and the speed of access to the mailbox.

Examples of MDA programs: postfix and dovecot.

IMAP and / or POP3 servers

IMAP and POP3 are the protocols used by mail clients, that is, any software for reading and extracting messages. Each protocol has its own difficulties; This article discusses some of their key differences.

IMAP is a more complex protocol that allows, among other things, support several customers to connect to an individual mailbox simultaneously. Email messages are copied to customers, and the original remains on the mail server.

A simpler POP3 protocol moves email messages to a mail client computer (as a rule, the default is the user's local computer).

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

Spam filters

The purpose of the spam filter is to reduce the number of incoming spam or unwanted mail in the mailbox mailboxes. To achieve this goal, spam filters apply spam detection rules that take into account various factors (sender of the message, content and much more), which allows you to evaluate the message. If this estimate exceeds the so-called SPAM LEVEL, the message is recognized as spam.

Spam filters can be applied for outgoing mail. This feature can be useful if the user's mail account was hacked: thus you can reduce the amount of spam transmitted using this mail server.

An open source filter is a popular open source filter.

Antivirus

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

Web Mail (or Webmail)

Many users need a webmail. Web Mail (in the context of working with the mail server) is an email client, accessible to users via a web browser (Gmail probably the most famous example). This component needs a web server (for example, Nginx, Apache) and can be started directly on the mail server.

Examples: RoundCube and Citadel.

Mail Server Support

After reading the server components of the email you want to install and configure, consider the following question: why support a mail server can be too time-intensive and require high time? There are routine maintenance tasks: updating the rules of antivirus and spam filter, support for all components up to date, etc.; But in addition, there are many other points.

Black lists

One of the common problems belonging to the support of the mail server is the need to keep it outside of black lists (which are also called DNSBL, Blacklists, or Blackhole Lists). Such lists contain IP addresses of mail servers that distribute spam or "waste" mail, as well as servers with incorrectly configured DNS records. Many mail servers subscribe to one or even several such black lists, after which the incoming messages are filtered depending on whether the mail server is entered by sending a message to the list (or lists). If the mail server enters the emergencies, its outgoing messages will be filtered (and some are blocked) before sending to the recipient.

As a rule, the mail server that fell into emergency can be excluded from this list. To do this, you need to find out the cause of the server to the blacklist and eliminate it. Then you need to trace the process of deleting the server from the lists and subscribe at least one of them.

Troubleshooting

Although most people use e-mail every day, not every one can eliminate malfunctions in this complex system. For example, what if the sent messages were not accepted by the recipient? The problem may arise due to improper configuration of one of the many components of the mail server (for example, due to the poorly configured spam filter from outgoing messages, or external factors like black lists).

How does email work?

Sending and receiving electronic letters are engaged specifically designed and highlighted mail servers. Before we start using Mail, these servers are configured completely different from other servers in the way. This, for example, SMTP.-Servers (OT. SIMPLE MAIL SERVER PROTOCOL - the main protocol of the letter transfer) and are mail clients or postal intermediaries (programs, within which we read and write emails, MUA. or Mail User Agent). But about the types of mail servers a little later.

How e-mail works. The path she passes.

Take two postal providers: server-1.Ru. and server-2.ru.. On them, respectively, two users were registered: A and B. In order to successfully reconciled the letter to the recipient B, and he accepted it, the following occurs. The scheme is quite simple:

Sender and sends a letter through your SMPT server-1.
Server-1, having received the task through the destination address using this very post mediator MUA., Accepted to search for server-2 in the domain portion of the address (RU, COM, RF ...). It does not matter where he is in Brazil or Mexico: the search will take a moment.
Having found it with the help of a domain name system (contacting it directly), the server-1 safely unscrews a letter "Meet" through the so-called postal exchanger MX. (mail Exchanger), he immediately begins to look for in its depths at the address of the registered user, keeping it for reading.

By finding a letter overtakes the addressee. The recipient receives access to the letter, receiving an alert from the server-2. The letter can be read through the network, POP or IMAP.

How does email work? What do these reductions mean?

All these are transmission protocols. At the same time, these are the types of mail servers. Their three:

SMTP servers - The most rapidate - transmit a letter using the mail client (by mail program: Outlook, Mozilla Thunderbird, etc.). For transmission used 25 by default.

Pop servers Already carry part of the software that allows the recipient to enter directly to the mail server. There, in a section-specifically assigned to this user (), it can work with correspondence. So, the protocol POP3Working through port 110 allows you to download your email client from the server all the letters received to your address. By the way, this is perhaps the only thing to what the protocol is capable of downloading.

Well IMAP server. - Software running on a computer-server that gives access to all user-created folders. With letters you can work in full, without downloading to your computer. The functionality prevails over POP3, but already requires additional memory and server processor resources (well, such tasks!). Data exchange goes through port 143.

Reference

To the postal address icon "@" "glued" thanks to the American engineer, Rai Tomlinson, who is the inventor of mail, which in the early 70s of the last century connected Arpanet users. " I was often asked why I chose the @ icon to designate mail. And only this sign and gives meaning the designation of mail. Indeed, in commercial English "AT SIGN" - @ - denotes a "unit of goods". I thought that @ perfectly can work as a pointer to the accessories and the user to the email server. That's how email is running now” .

By the way, to say, the moment of modern use of the "Doggych" icon is a year 1536. It was then that the Spanish merchant used @ in the accompanying note when specifying the price of goods sent to Rome. But it is reliably known that the symbol appeared much older. So, it was documented by its use in the document dating from the middle of the 14th century AD.

How e-mail works. What makes a letter from?

The letter consists of two parts: the one that is interested in sending and reading this message, and the one that interests us with you no less. That is, the descriptive part contains the address (from where and where). They are called respectively body letter and letter heading.

FROM tel Email everything is clear. It includes the composition of the message and any attached data in countless formats. Previously, the letter understood a limited number of encodings, but, like other network services, he was soon taught to understand even multimedia content.

But on title Few people pay attention to the whole. No, this is not an envelope of paper letters with a return address. Not only at least. This is a structured information about the sender, the recipient, attachment objects, the date of shipment, servers and services, which took part in the delivery, etc. It is very useful to view if the writing is suspicious.

Reference

In each language The name of the symbol "@" is different. In English, it is pronounced as "at Sign" - "Piece" and is very common in a commercial language. Many European countries, and, naturally, Japan took about-English concept, and in different interpretations, anyway, the @ icon continues to mean "a piece". In Russian, the concept of "dog" passed due to an unknown programmer / there, who worked in the USSR on the DVK - Dialogue computing complex. Someone noticed the similarity of the icon with his head and the tail of the dog and rushed ... We now enjoy similar concepts now and the states of the former union of the republics. Dog characteristics@ Posses in Ukraine, Kazakhstan and Uzbekistan. But Serbs, Romanians and Slovaks saw in it similarity with monkey ...

How e-mail works. We study the title of the letter. Spam?

You will need this information if you want to dug up in the direction of how email is running on your specific example. What can be learned from the header?

It doesn't matter what way to read letters you use. No matter what client is installed on your computer. And where the letters are stored: if his client shakes you on your computer (I do not know why?) Or they are stored on the mail server. The title from us is not going anywhere.

Consider an email header on the example. My email client - Thunderbird from Mozilla.. Long living program, I like it. Select a letter and open it with the right mouse:

I do not know from where you are viewing, but for each service or the client, there is necessarily a toolbar that allows you to work with a letter, showing its source code. In my client it is here, right-top:

That's what you will see:

How does an email work work? What can I learn from this?

We strip in the header of the letter important information for us ...

Field Return-Path. ("Track Home") empty is suspicious. Near this inscription should be the address of the sender, if you want to send the return letter. The empty field means only one thing - spam. But even the filled field with a certain address does not mean that they write specifically and in the case. The alphanumeric character of Belibard before the @ icon is often also spam indicator.

Tags Recieved.: Look at them. In your letter these tags may be several. A large number of them means that the sender tried to hide his traces. I have only one such a line. Tags are read (if there are several of them) from the bottom up - just as a letter and it was sent to you. The characteristic feature of the spam is the lack of an important characteristic: tag with.It is he who informs the protocol that you need to send a letter. I have a tag on the spot: the spammer is completely "blurred" or the owner of the mailbox does not suspect that letters go with his email. RETURN-PATH,however, empty ...

Today, less and less email servers allow open transmission of messages. And the spammer has to use our mail server, that is, the recipient of the letter. Therefore, do not be surprised that a considerable number of spam gets from the mailbox of your Internet provider ... there is some kind sofkon.ru.... what a hosting ...

Field Message-ID- a unique email identifier. Anyone. Its number to us for nothing, but the server name after the "doggy" must match the name in the mailbox. It's all right here, but let's check the performance of the host himself. Just score in the address bar sofkon.ru.:

Predictable ... Well. Let's now look at the appearance appearing three times in the letter (linked brown - does not deserve another color). Before sending an angry writing there, check for the sake of interest this drawer for reality. This will help us a very useful service. 2IP:

https://2ip.ru/mail-checker/

We enter the address and:

Yes, we have not doubted already ...