The Simple Mail Transfer Protocol (SMTP) is a protocol of the network protocol family that is used to exchange e-mails in computers or other devices. It is primarily used for feeding and forwarding emails. Other specialized protocols such as POP3 or IMAP are used to fetch and read messages. SMTP servers traditionally accept connections on port 25.
SMTP server was first described in RFC 821; The latest update in RFC 5321 (2008) includes a scalable extension called ESMTP. Currently, the SMTP server's protocol as a rule also means its extensions.
Electronic mail servers and other message forwarding agents use SMTP server to send and receive mail messages, client-level email client applications typically use SMTP server only to send messages to the mail server for relay.
E-mail is represented by the mail client for the mail server using SMTP over TCP port 587. The MSA delivers mail to its message transfer agents. Often these two agents are simply different samples of the same software running with different parameters on the same device. Local processing can be carried out either on a separate machine or shared between different devices.
The processes involved share files and SMTP server is used to forward the message internally then each host configured to use the next device as an intermediate host. Each process is an MTA in itself and that is an SMTP server.
The border MTA must find the target host. It uses the domain name system to search for mail exchanger records of the recipient's domain. The returned mail MX record contains the name of the target host. The MTA connects to the exchange server as an SMTP server's client.
As soon as the MX target receives the incoming message and SMTP server sends it to the mail delivery agent for local delivery of the message. MDA provides the ability to save messages in an appropriate mailbox format. Again reception of mail can be carried out by several or one computer - the image shows the two closest boxes for each case. MDA can deliver messages directly to storage or send them over the network using SMTP server or any other means then including the Local Mail Transfer Protocol and a derivative of SMTP designed for this purpose.
After delivery to the local mail server, the message is stored for batch search on authenticated mail clients. The message is retrieved by end-user applications using SMTP server's IMAP, which facilitates access to messages and manages stored mail or using POP that which usually uses the traditional mbox file format or proprietary systems like Microsoft Exchange / Outlook or Lotus Notes / Domino. Network mail clients can use any method, but the search protocol often does not meet official standards.
SMTP server defines the transmission of a message and not its content. Also it sets the message shell and its parameters and not the header or body of the message itself. STD 10 and RFC 5321 define SMTP server, while STD 11 and RFC 5322 define a message and officially called the Internet Message Format.
SMTP server is a text-based protocol requiring connection, by which the sender of the message communicates with the recipient by issuing command lines and receiving the necessary data through a reliable channel and usually acts as a TCP Connection. An SMTP session consists of commands sent by an SMTP client and corresponding responses from an SMTP server. When the session is open then the server and the client exchange its parameters. A session can include zero or more SMTP operations.
MAIL FROM - sets the return address. This is the address for returned emails.
RCPT TO - sets the recipient of this message and this command several times given one for each recipient. These addresses are also part of the shell.
DATA - to send the message text. This is the very content of the letter and opposed to its shell. It consists of the message header and message body that separated by an empty string. it is a group of commands and the SMTP server responds twice: the first time to the DATA command itself and to notify that it is ready to receive text; and a second time after the end of the data sequence to accept or decline the entire letter.
In addition to the intermediate responses for the DATA command, each SMTP server response can be positive or negative. Later in turn can be permanent or temporary. Failure of the SMTP server to send a message - a permanent error; Then client must send the returned letter. After resetting - a positive response, the message will most likely be rejected. Also, the server can report that additional data is expected from the client.
The initial host can be either the end-user mail client or the message forwarding agent on the SMTP server. For example the server acts as a client in the corresponding session to relay the message. Fully functional servers support message queues for retransmission of messages in case of errors.
MUA knows the SMTP server for outgoing mail from its settings. An SMTP server acting as a client, that is, forwarding messages, determines which server to connect to by viewing the MX record resource DNS for each recipient's domain . If an MX record is not found, compatible MTAs revert to a simple A record . Forwarders can also be configured to use Smart host.
An SMTP server acting as a client establishes a TCP connection with the server on port 25 designed for SMTP. The MUA must use port 587 to connect to the Message Submission Agent. The main difference between MTA and MSA is that SMTP authentication is required only for the latter in SMTP server.
SMTP server is just a delivery protocol. It cannot demand take messages from a remote server. POP and IMAP protocols have been developed for mail retrieval and mailbox management. SMTP provides the ability to start processing a message queue on a remote server and the requesting system can receive all messages sent to it. POP and IMAP are preferred when the user's computer is not always on or is temporarily connected to the Internet.
ODMR is a standardized in RFC 2645 SMTP server's extension that allows relay messages to authenticated users.
The mail client must know the IP address of the SMTP server, which is specified as part of the configuration (usually in the form of a DNS Name ). The server will deliver outgoing messages on behalf of the user.
Outgoing server access restrictions
SMTP server's administrators need to control which clients can use the server. This allows them to deal with abuses such as spam. Two solutions are commonly used:
In the past, many systems introduced restrictions on the location of the client, allowing only those whose IP address was among the administrators controlled.Modern SMTP servers usually offer an alternative system that requires client authentication to gain access.
The Internet service provider's SMTP server will not allow access to users outside the provider's network. More precisely, the server can only allow those users whose IP address is provided by this provider, which is equivalent to requiring an Internet connection using this provider. A mobile user can often end up on a network other than the network of his provider, and therefore messages will not be sent.
This system has several varieties. For example an organization's SMTP server can only provide access to users on the same network, blocking other users. That SMTP server can also perform a series of client IP address checks. These methods were commonly used by organizations and institutions for internal server use. Most of them now use the Authentication methods described below.
Restricting access to specific addresses using SMTP server administrators can easily determine the address of any attacker. If a user can use different providers to connect to the Internet, this type of restriction becomes impractical, and changing the configured address of the outgoing mail SMTP server is impractical. It is highly desirable to be able to use such information about client settings that does not need to be changed.
Instead of the location restrictions described earlier, modern SMTP servers typically require user authentication before gaining access. This system, being more flexible, supports mobile users and provides them with a fixed choice of a configured Outgoing mail server.
A SMTP server that is accessible over a wide network and does not provide these types of access restrictions is called an open relay. Now these servers are considered bad form.
SMTP server's Ports SMTP server administrators choose which port clients will use to relay outgoing mail - 25 or 587. Specifications and many servers support both ports. Although some servers support port 465 for secure SMTP, it is preferable to use standard ports and ESMTP commands if you need a secure session between the client and server.
Some SMTP servers are configured to reject all relays on port 25. Authenticated users on port 587 are allowed to forward messages to any valid address.
Few providers intercept port 25, redirecting traffic to their own SMTP server, regardless of the destination address. Thus, their users cannot access the server outside the provider network on port 25.
Also some SMTP servers support authenticated access through an additional port other than 25, allowing users to connect to them even if port 25 is blocked.
A typical example of using SMTP server to send an SMTP message to two mailboxes located in the same mailbox and reproduced in the following exchange session. for example, the conversation parts are the prefixes S-server and C-client; these tags are not part of the exchange.
If the sender of the message (SMTP client) creates a reliable channel of communication with the recipient of the message (SMTP server), the session opens with a greeting word from the server, usually containing the name of its fully qualified domain (FQDN), in this case SMTP.example.com. The client initiates its own dialogue by responding with a command HELOby identifying itself in the command parameter with its own FQDN.
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