Electronic mail on the Internet Principles of organization. How email works
System Email
The email system consists of three components:
user agent - allows users to read and compose messages.
transport agent - forwards messages from one machine to another.
delivery agent - places messages in the mailboxes of recipient users.
User agents.
Programs that allow users to read and compose mail messages Examples of these agents include Internet program Mail in Windows 95, mail command in UNIX.
The very first user agent was /bin/mail, developed by AT&T. Now there are several programs of this class. Additionally, there are user agents with graphical interface user. There is also a standard that defines the inclusion of multimedia objects in email messages. It's called MIME (Multipurpose Internet Mail Extensions). Many user agents support this standard.
Transport agents.
Programs that accept mail from a user agent interpret user addresses and redirect mail to the appropriate computers for subsequent delivery. In addition, the transport agent receives incoming mail from other transport agents. The transport agent is working SMTP protocol(Simple Mail Transport Protocol) which is defined in RFC821.
Several transport agents have been developed for the UNIX OS (MMDF, zmailer, smail, upas and others), but the most powerful, most flexible and most widespread is sendmail.
The sendmail program is a transport agent, a linking program between user and delivery agents. For the Internet, it is also a delivery agent. The sendmail program performs the following tasks:
managing messages after they have left the user's fingers;
parsing recipient addresses;
selection of the appropriate delivery or transport agent;
converting addresses into a form understandable to the delivery agent;
necessary reformatting of headings;
transmission of the converted message to the delivery agent.
The sendmail program also generates error messages and returns messages that cannot be delivered to the sender.
Delivery agents.
Programs that accept mail from a transport agent and deliver it to the appropriate users. Mail can be delivered to a specific person, to a mailing list, to a file, to a program, etc. To service each type of recipient, a separate mail agent-- delivery agent local users. Spop is a delivery agent for users of remote machines who use UUCP (UNIX to UNIX Owl) or POP (Post Office Protocol) to receive mail.
The /bin/mail program is a delivery agent for local users, and the uux and spop, fetchmail programs are delivery agents for users of remote machines who use UUCP or POP, IMAP services to receive mail. The /bin/sh program is a delivery agent for mail that is sent to a file or program.
Addressing in the email system
There are two types of email addresses: route-dependent and route-independent. When using the first addressing method, it is required that the sender knows the intermediate machines through which the message must pass in order to get to its destination. The second type of address simply indicates the destination. UUCP addresses are route-dependent, but Internet addresses are (usually) route-independent.
An Internet email address has the following format: user@machine, where the @ sign separates the user name from the machine designation. Mail is delivered to the user's mailbox on the user's machine.
Example email address This email address protected from spam bots. To view it, you must have JavaScript enabled
In the example under consideration, yvv is the subscriber’s identifier, usually composed of the initial letters of his last name, first name, and patronymic. What is to the right of the @ sign is called a domain and uniquely describes the location of the subscriber. The components of a domain are separated by dots. The rightmost part of the domain, as a rule, indicates the country code of the recipient - this is the domain top level. The country code is approved by the international ISO standard and is given in Appendix 1. In our case, ru is the code Russian Federation. However, a network designation can also appear as a top-level domain. For example, in the USA, where there are networks uniting the highest educational establishments or government organizations, the abbreviations edu - Educational institutions (for example, cs.berkeley.edu), gov - Government institutions and others are used as top-level domains (see Appendix 1).
The next subdomain - saratov in our case - is uniquely defined within the top-level domain. It is not difficult to guess what the city code means - Saratov. Totality components the domain saratov.ru is called a second-level domain. Second-level domain abbreviations are defined in accordance with the rules adopted by the top-level domain.
Third level domain - softpro.saratov.ru. In our case, the third-level domain includes the name of the SoftPRO Group company. The rules for forming names within third-level domains are a private matter for second-level domains.
UUCP address consists of a list of machines through which the message must pass on the way to its destination. List elements separate exclamation marks. For example, in the UUCP email address: mcvax!uunet!ucbvax!hao!boulder!lair!evi
the destination is the machine lair and the destination is the user evi. Each machine in the chain has a direct UUCP connection to machines that are on the network before and after it. For example, the ucbvax machine must have connections to the hao and uunet machines. Chains of UUCP addresses can be very long, but now that the Internet is widely used, you rarely see real huge ones. When email was primarily based on UUCP, administrators were forced to remember a list of computers over fairly large areas core network UUCP.
In Internet email format, the above address will look like evi@lair, or more precisely - This e-mail address is being protected from spambots. To see it, you must have JavaScript enabled. Combined addresses are also allowed. For example, if a UUCP system connects the machine lair to the rest of the colorado.edu domain, the address might look like this: lair! This e-mail address is being protected from spambots. To view it, you must have JavaScript enabled
Mail aliases
Aliases allow system administrator and forward mail to individual users. They can be used to set up mailing lists (that include multiple recipients), to forward mail between machines, and to allow users to be addressed by multiple names.
Aliases can be defined:
in the user agent configuration file;
in the system-wide alias file /etc/aliases;
V user file forwards ~/.forward.
The email system first looks for aliases in the user agent configuration file, then in the aliases file, and finally in the user forwarding file.
Here are some examples of aliases defined in the aliases file:
evi: evi@mailhub
authors: evi,garth,scott,trent
The first line states that mail arriving at nemeth should be delivered to user evi on local machine. The second is that all mail addressed to evi should be delivered to the mailhub machine. Finally, the third line specifies that mail addressed to authors should be delivered to users evi, garth, scott, and trent. Recursion is supported so that mail sent to nemeth eventually ends up at evi@mailhub.
In addition to lists of users, aliases can refer to:
the file to which messages should be added;
command to whose input messages should be sent.
Mail message format
In order to email has reached its addressee, it is necessary that it be formatted in accordance with international standards and have a standardized email address. The generally accepted format of the message is determined by a document called " Standard for the Format of ARPA - Internet Text messages", abbreviated as Request for Comment or RFC822, and has a header and the message itself.
Let's look at an example email message:
Received: from CS.ORST.EDU by fuug.fi with SMTP id AA15539 (5.65+/IDA-1.3.5 for This e-mail address is being protected from spambots. You need JavaScript enabled to view it) ; Thu, 20 Dec 90 08:19:05 +0200
Received: from jacobs.CS.ORST.EDU by CS.ORST.EDU (5.59/1.15) id AA19981; Wed, 19 Dec 90 22:19:59 PST
From: Harry Brooks< Этот адрес e-mail защищен от спам-ботов. Чтобы увидеть его, у Вас должен быть включен Java-Script >
Message-Id:< Этот адрес e-mail защищен от спам-ботов. Чтобы увидеть его, у Вас должен быть включен Java-Script >
To: This e-mail address is being protected from spambots. To view it, you must have JavaScript enabled
Subject: Re: wondering if you attended?
Hi! Check of communication. If the message has passed normally came confirmation.
The message consists of the text that you want to send to the recipient, and a header, which is added at the beginning of the message, separated from the text by a blank line, and contains several lines necessary information about this message: date of departure, address, return address, subject of the message, and others.
Here the first fourteen lines make up the title. Note that each of the header lines has the form: title: text
The header line names are deciphered as follows:
Received: mark of passage through the machine (postmark). Our letter has five such marks, which means that along the way it passed through five cars, and each of them indicated when it passed.
Date: date and time the letter was sent; they are specified in a standard format, since most postal systems They can sort messages by time if you ask.
From: sender's name and return address<отделен угловыми скобками>.
Message-Id: internal message identifier; assigned by the sender's postal service. Each letter is assigned a unique - unique in the world - identifier. It can be used to link to a letter, like an outgoing number.
To: recipient's address
Subject: subject of the message. The Re: mark means that this message is a response (from the word reply) to another message. The original message and the response have the same Subject: line. When the author responded, the postal service automatically took the subject line from the original message. This is convenient when there is a long conversation on one topic. You can request that the mail service sort messages by topic and refresh your memory of previous phrases in this conversation.
When composing a message, try to indicate in this line the title that is short and as informative as possible. Not everyone will read a message with a headline like “Do you remember once you told me...”?
Status: message status; Your postal service marks for itself that you have already read the message, so that it is not offered to you a second time as new.
There are several other types of header lines. Not all of them have to be there. Some lines are added automatically by postal services. (Received: Date:), others are set by the author of the letter (To:, Subject:). We will dwell in detail on how to indicate the address in a message so that the postal services understand it and deliver the letter to its destination.
You can enter the message text and create a header using one of the email message editors.
The message itself is usually text file free form. When transmitting non-text data ( executable program, graphic information) message recoding is applied, which is performed by appropriate software.
Sending faxes
IN Lately appeared on the Internet new opportunity- send and receive faxes over a network using a computer. You can send an order to send or receive a fax. A regular e-mail, properly formatted, is compiled and sent to the address of the computer node that handles fax operations. The text of this letter will be delivered as a fax to the recipient's fax machine.
Fax software allows you to convert data in various formats to the format of fax machines. For example, the Quick Link II Fax program allows you to transfer the following data to fax machines and other fax modems: text, files in TIFF, IMG formats prepared by the GEM Artline or Ventura Pablisher program, BMP from Microsoft Windows, CUT from Dr.Halo and PCX from Paintbrush.
INTRODUCTION
The messaging system under the elegant name “e-mail” is one of the fastest (and most accessible) means of communication on the Internet and local networks and, accordingly, one of the most popular. Oddly enough, it is also one of the oldest inventions of civilization at the stage of development of telecommunications.
What is it, how can it be used.
We know what mail is. These are traditional means of communication that allow the exchange of information, at least, two subscribers.
Two types of communication - postal and telephone - have become traditional for us, and we already know their advantages and disadvantages well. What is email?
The main attraction of email is its speed. However, there are other benefits that are not so widely known. The phone also provides almost instantaneous access, but studies have shown that about 75% of phone calls end in failure. E-mail has the same access speed as the telephone, but does not require the simultaneous presence of both subscribers at different ends of the telephone line. In addition, she leaves a written copy of the message, which can be saved or passed on. Moreover, a letter can be sent to several subscribers at the same time. Using modern e-mail services, you can transmit not only written messages, but information of any kind: photographs, videos, programs, etc.
And all this is guaranteed to be sent anywhere globe for a few minutes.
PRINCIPLES OF WORKING WITH EMAIL
The birthday of email is October 21, 1969. Legend has it that on this day the world's first email was sent (and therefore received). It consisted of no less than two letters. After its transfer, one of the computers participating in the experiment crashed.
All actions used when writing and sending classic mail messages are reflected in the operations that we perform when preparing and sending emails. We type the text on the keyboard, indicate the address of the recipient (or recipients), then put the letter in an electronic envelope, place it in a mailbox located on the mail server, from where it is picked up by the postal service and delivered to the recipient's mailbox.
As in other types of computer communications, the main point in this entire system is the interaction of two programs - the mail server and the mail client.
Mail clients are the very programs that are at the disposal of every user who sends or receives mail. There are a great variety of such programs. As a rule, they are distributed free of charge or almost free, and anyone can choose something to their liking.
All email clients have approximately the same interface. This similarity also allows us to visualize how email works, regardless of which specific program you use.
You can send and receive not only text, but also any files in emails. Message format that allows you to send with a letter various files, in number of HTML and media data is called MIME. This acronym stands for Multi-Purpose Internet Mail Extensions. As an extension to the basic SMTP (Simple Mail Transport Protocol) mail format. If SMTP only allowed transfer text information, then MIME gave people the opportunity to greatly enrich the language of mail on the Internet.
The abbreviation SMTP stands for Simple Mail Transport Protocol. This protocol defines the rules for sending and receiving email messages over the Internet. Due to some functional limitations (it was developed quite a long time ago), SMTP is not able to accumulate messages on the recipient side. Therefore, when receiving mail, we are dealing with another one of the mail protocols.
The POPS protocol (Post Office Protocol 3) is a protocol for receiving messages. In accordance with it, mail is received by the server and accumulated on it. The program - an email client - periodically checks mail on the server and downloads messages to local computer.
We send mail using SMTP and receive mail using POP3. That's why in the process of creation account mail you are asked to enter the names as SMTP server, and the POPZ server.
It is not necessary to use POP3 to receive mail. There is another protocol that allows you to accumulate messages on a server and download them to your local computer. It's called IMAP (Internet Message Access Protocol).
IMAP allows you to manipulate files on the mail server various actions, unlike POP3, which simply transfers messages to your computer and deletes them from the server. In this case, messages are stored on the server, and their state is not affected in any way by your actions on the local computer.
An email address generally looks something like this: [email protected]
The word that appears on the left side of the address before the @ sign) is the name of the addressee.
When you obtain an email address - from your ISP or local network enterprise - you can determine the type of your address (more precisely, the left part of it).
What's to the right of the @ sign is the domain name of the server where your messages are stored.
If you need to write a response to someone who sent you an email, you can quickly create a reply message. This opportunity is provided in every mail program. This message will differ favorably from empty message the following parameters: it will already contain the required address in the To (To) field; The subject of such a letter will appear in the Subject field in the form Re: the subject of the original (that is, sent to you) message. Finally, the text of the original message can be inserted into the reply message and highlighted as you specify. Typically, you can create a reply message using the Reply button or menu command.
If you are not the only recipient of the letter, it is often necessary to send a reply message not only to the sender, but also to everyone who received the letter besides you. Typically, this is done using the ReplyAll button or menu command.
The forwarding option is provided so that you can forward the letter you received to someone else. For example, you can forward an email sent to you to another person, or resend an email from your Sent Items folder. The text of the original message will be inserted into the forwarded message. Typically, the Forward button or menu command is used for forwarding.
The Internet, as a relatively new communication medium, has given rise to some new rules for written communication - rules that no one thought about following in the past. There were, of course, some basic concepts(such as a respectful attitude towards the interlocutor), which reasonable people followed in conversation and correspondence, - however, things sometimes did not go beyond pretentious and vague appeals.
Meanwhile, new times dictated new realities. With the widespread use of e-mail, the contours of online communication norms began to emerge clearly, outlining the unwritten boundaries of what is acceptable. There was such a concept as Netiquette (translated as “netiquette”), that is, network etiquette.
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Email is one of the most important information resources Internet, the most widespread means of electronic communications. Any Internet user can get their own mailbox on the network. Considering that via the Internet you can receive or send messages to another two dozen international computer networks, some of which do not have on-line service at all, it becomes clear that mail provides opportunities in some sense even broader than just information Internet service. A good example can serve as access to archives of the BITNET network - documents and teleconferences maintained on BITNET list servers (LISTSERVER). Gateways to FIDO, Relcom, Sprint and the like networks are also publicly available.
PRINCIPLES OF ORGANIZATION
The simplest solution for organizing messaging suggests the following. Let us assume that user A has host A connected to some data transmission network (DTN). User B with host B is a subscriber of the same network. Then, to transmit messages from A to B, it is necessary to organize a messaging protocol (or simply, mail) and this protocol must be implemented on both hosts. Let's assume that SPD does not directly provide such a protocol, then A and B bear the entire burden of receiving/transmitting mail. If the user is a subscriber to several networks, then everything necessary will have to be installed on his computer software, implementing all kinds of protocols. The load will increase if, in addition to software interfaces hardware will need to be installed.
The real problems arise when it turns out that the remote host (B) is unreachable for a number of reasons:
· he could have changed the host name and did not notify the correspondent about this;
· perhaps its host is simply disabled or simply
· on this moment there is no direct route from A to B.
In all these cases, the mail will not reach host B. In the best case, user A will receive a notification that it is impossible to reach host B, and in the worst case, the message will simply be lost in the SPD.
Thus, for the reliable delivery of messages, an approach has been developed in which Electronic mail is in many ways similar to the regular postal service. Correspondence is prepared by the user at his workplace either by a mail preparation program or simply by regular text editor. The user must then call the mail program (usually the mail preparation program calls the mail program automatically). Standard program sending to Unix systems is the sendmail program. Sendmail works as a postal courier that delivers regular mail to the post office for further distribution. On Unix systems, sendmail is itself a communications department. She sorts the mail and sends it to the recipients. For users personal computers who have mailboxes on their machines and access mail servers over dial-up phone lines may need to additional actions. For example, users of the Relcom mail service must run the uupc program, which delivers mail to mail server.
To transmit email on the Internet, a special Simple Mail Transfer Protocol (SMTP) has been developed, which is an application-level protocol and uses a transport TCP protocol. However, the Unix-Unix-Copy (UUCP) protocol is also used in conjunction with this protocol. UUCP is good to use telephone lines communications. Most Relcom email users actually use this protocol to deliver mail to the site. The difference between SMTP and UUCP is that when using the first protocol, sendmail tries to find the recipient machine and establish online communication with it in order to transfer mail to its mailbox. When using SMTP mail reaches the recipient's mailbox in a matter of minutes and the time it takes to receive the message depends only on how often the recipient checks his mailbox. When using UUCP, mail is transmitted on a stop-go basis, that is, the mail message is transmitted along a chain of mail servers from one machine to another until it reaches the recipient machine or is rejected due to the absence of the recipient subscriber. On the one hand, UUCP allows you to deliver mail over poor telephone channels, since it is not necessary to maintain the line during the entire delivery from the sender to the recipient, and on the other hand, it can be frustrating to receive a message returned a day after it was sent due to the fact that an error was made in the username. Therefore, UUCP is most often used to access an organization's mail server from home via dial-up communication channels, and the organization's mail server then sends the message using the SMTP protocol.
New opportunities that have opened up in our age of electronic communications, local and global electronic networks, allow people to abandon the usual form of correspondence through standard letters And postal services. Email(E-mail) is one of the main Internet services that allows you to conduct intensive correspondence with a subscriber located anywhere in the world almost in real time.
For reference email correspondence appropriate software is required. And, of course, so powerful Information system Like Microsoft Outlook, it has everything you need for intensive email work. Outlook is capable of conducting full correspondence with the subscriber: receiving, sending and forwarding email messages, automatically adding subscribers to address book, sort the information received - and this is far from a complete list of Outlook's capabilities as an email application.
This chapter will talk about setting up the services necessary for receiving and sending messages, creating, editing and formatting messages, and will also discuss in detail working with folders Inbox(Inbox) Sent(Sent) Outgoing(Outbox) Drafts(Drafts).
Basic principles of working with email
Before you start detailed description, we will present a number of fundamental concepts and talk about the basic principles of working with email. Working on the Internet and, in particular, with e-mail is associated with such key concepts as server, client, protocol, email address, data transfer.
Server- This special program, located on a remote machine and providing its services to client programs. In the case of email, the server is the remote machine on which the mailbox with your messages is located.
Client - it is also a special program that uses the services provided by the server. In our case, this program is Outlook, which allows you to view messages received in your mailbox on the server.
Protocol - this is a set of rules that determine the algorithm for transmitting data from the server to the client and vice versa.
Email address - a unique identifier that identifies your mailbox where messages arrive.
After the above definitions, I want to talk about general principles work with e-mail, which will allow you to understand the process of sending and receiving messages from the inside, demonstrating them using the example of regular mail.
My post office number is 332 (index - 198332), this is where all letters, newspapers and magazines addressed to me and other subscribers of this post office are received. Each incoming letter, in addition to the index common to all subscribers, has additional characteristics: Street, house, flat. Based on these characteristics, the postman clearly determines where the correspondence should be delivered, namely, my mailbox. A similar situation occurs with sending my letters to someone. I put them in the mailbox, the postman takes them and takes them to the post office, from which it is delivered in the appropriate way (for example, by airmail) to the addressee, who also has mailing address, compartment and drawer.
Now I will translate the described situation into the language of e-mail. Post office there is nothing more than a server on which incoming and outgoing messages are stored and which has a unique address, for example MAIL.RU (read: 198332). An email address is an extension of the server address, which uniquely determines who owns the incoming message, for example: [email protected](read: street_house_apartment@198332). By mailbox acts as a client application, in our case Microsoft Outlook, installed on your computer, where messages ultimately arrive. Postman, in the form of email, is nothing more than a protocol through which messages are transmitted from the server to the client and vice versa. Feedback looks exactly the same: if you send a message to another user, you need to know their email address (for example, [email protected]), after sending a message to this address, it is moved to your MAIL.RU server, after which it is sent to the YAHOO.COM server, and then finally to the addressee [email protected].
I hope these definitions and explanations help you as you work with email. The process of working with email is shown schematically in Fig. 9.1.
Rice. 9.1. How email works