.

SMTP is a relatively simple, text-based protocol, in which one or more recipients of a message are specified (and in most cases verified to exist) along with the message text and possibly other encoded objects. The message is then transferred to a remote server using a series of queries and responses between the client and server. Either an end-user's e-mail client, a.k.a. MUA (Mail User Agent), or a relaying server's MTA (Mail Transport Agents) can act as an SMTP client.
An e-mail client knows the outgoing mail SMTP server from its configuration. A relaying server typically determines which SMTP server to connect to by looking up the MX (Mail eXchange) DNS record for each recipient's domain name. Conformant MTAs (not all) fall back to a simple A record in the case of no MX (relaying servers can also be configured to use a smart host). The SMTP client initiates a TCP connection to server's port 25 (unless overridden by configuration). It is quite easy to test an SMTP server using the netcat program (see below).
SMTP is a "push" protocol that cannot "pull" messages from a remote server on demand. To retrieve messages only on demand, which is the most common requirement on a single-user computer, a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN. It is possible to receive mail by running an SMTP server. POP3 became popular when single-user computers connected to the Internet only intermittently; SMTP is more suitable for a machine permanently connected to the Internet.
A simple aid to memory is "Send Mail To People."

Outgoing mail SMTP server

An e-mail client requires the name or the IP address of an SMTP server as part of its configuration. The server will deliver messages on behalf of the user. This setting allows for various policies and network designs. End users connected to the Internet can use the services of an e-mail provider that is not necessarily the same as their connection provider (ISP). Network topology, or the location of a client within a network or outside of a network, is no longer a limiting factor for e-mail submission or delivery. Modern SMTP servers typically use a client's credentials (authentication) rather than a client's location (IP address), to determine whether it is eligible to relay e-mail.
Server administrators choose whether clients use TCP port 25 (SMTP) or port 587 (Submission), as formalized in RFC 4409, for relaying outbound mail to a mail server. The specifications and many servers support both. Although some servers support port 465 for legacy secure SMTP in violation of the specifications, it is preferable to use standard ports and standard ESMTP commands[6] according to RFC 3207 if a secure session needs to be used between the client and the server. Some servers are set up to reject all relaying on port 25, but valid users authenticating on port 587 are allowed to relay mail to any valid address. A server that relays all e-mail for all destinations for all clients connecting to port 25 is known as an open relay and is now generally considered a bad practice worthy of blacklisting.
Some Internet service providers intercept port 25, so that it is not possible for their users to send mail via a relaying SMTP server elsewhere using port 25; they are restricted to using the ISP's SMTP server only. Some independent SMTP servers support an additional port other than 25 to allow users with authenticated access to connect to them even if port 25 is blocked. The practical purpose of this is that a travelling user connecting to different ISPs otherwise has to change SMTP server settings on the mail client for each ISP; using a relaying SMTP server allows the SMTP client settings to be used unchanged worldwide.

Sample communications

After establishing a connection between the sender (the client) and the receiver (the server), the following is a valid SMTP session. In the following conversation, everything sent by the client is prefixed here with "C: " and everything sent by the server with "S: "; this prefix is not part of the conversation. On most computer systems, a connection can be established using the netcat command (or telnet if netcat is not available) on the client machine, for example:
telnet smtp.example.com 25
which opens a TCP connection from the sending machine to the MTA listening on port 25 on host smtp.example.com. By convention, SMTP servers greet clients with their fully-qualified domain name. In this example, the client computer (relay.example.org) has already determined that "smtp.example.com" is a mail exchanger for the example.com domain by doing a DNS lookup of example.com's MX records. Note that a carriage return and a line feed character (not shown) are required at the end of each line; in a manual Telnet session they are both normally generated by pressing the Enter or carriage return key once.
S: 220 smtp.example.com ESMTP Postfix
C: HELO relay.example.org
S: 250 Hello relay.example.org, I am glad to meet you
C: MAIL FROM:
S: 250 Ok
C: RCPT TO:
S: 250 Ok
C: RCPT TO:
S: 250 Ok
C: DATA
S: 354 End data with .
C: From: "Bob Example"
C: To: Alice Example
C: Cc: theboss@example.com
C: Date: Tue, 15 Jan 2008 16:02:43 -0500
C: Subject: Test message
C:
C: Hello Alice.
C: This is a test message with 5 headers and 4 lines in the body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
{The server closes the connection}
In this example, the e-mail is sent to two mailboxes on the same SMTP server: once for each recipient listed in the "To" and "Cc" headers; if there were any in a "Bcc" list, which are not included in any headers, there would have been additional "RCPT TO" commands for those recipients as well. If the second recipient had been located elsewhere, the client would QUIT and connect to the appropriate SMTP server once the first message had been queued. Note that the information the client sends in the HELO and MAIL FROM commands can be retrieved in additional headers that the server adds to the message: Received and Return-Path respectively.
Although optional and not shown above, many clients ask the server which SMTP extensions the server supports, by using the EHLO greeting to invoke Extended SMTP (ESMTP) specified in RFC 1870. These clients fall back to HELO only if the server does not respond to EHLO.
Modern clients may use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted. Older clients and servers may try to transfer huge messages that will be rejected after wasting the network resources, including a lot of connect time to dial-up ISPs that are paid by the minute.
Users can manually determine in advance the maximum size accepted by ESMTP servers. The user telnets as above, but substitutes "EHLO host.example.org" for the HELO command line.
S: 220-smtp2.example.com ESMTP Postfix
C: EHLO bob.example.org
S: 250-smtp2.example.com Hello bob.example.org [192.0.2.201]
S: 250-SIZE 14680064
S: 250-PIPELINING
S: 250 HELP
Thus smtp2.example.com declares that it will accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). Depending on the server's actual resource usage, it may be currently unable to accept a message this large. In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction.

Security and spamming

Main article: Anti-spam techniques (e-mail)
One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.
Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 was one such proposal for replacement.
Spam is enabled by several factors, including vendors implementing broken MTAs (that do not adhere to standards, and therefore make it difficult for other MTAs to enforce standards), security vulnerabilities within the operating system (often exacerbated by always-on broadband connections) that allow spammers to remotely control end-user PCs and cause them to send spam, and a lack of "intelligence" in many MTAs.
There are a number of proposals for sideband protocols that will assist SMTP operation. The Anti-Spam Research Group (ASRG) of the Internet Research Task Force (IRTF) is working on a number of E-mail authentication and other proposals for providing simple source authentication that is flexible, lightweight, and scalable. Recent Internet Engineering Task Force (IETF) activities include MARID (2004) leading to two approved IETF experiments in 2005, and DomainKeys Identified Mail in 2006.

Other protocols for e-mail

E-mail is "handed off" (pushed) from a client (MUA) to a mail server (MSA), usually using Simple Mail Transfer Protocol or IMAP. From there, the MSA delivers the mail to an MTA, usually running on the same machine. The MTA looks up the destination(s)'s MX records with a DNS lookup, and begins to relay (push) the message to the server on record via TCP port 25 and SMTP. Once the receiving MTA accepts the incoming message, it is delivered via a mail delivery agent (MDA) to a server which is designated for local mail delivery. The MDA either delivers the mail directly to storage, or forwards it over a network using either SMTP or LMTP, a derivative of SMTP designed for this purpose. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Generally speaking, mail retrieval (pull) is performed using either a type of online folders (e.g. IMAP 4, a protocol that both delivers and organizes mail) or the older single repository format (e.g. POP3, the Post Office Protocol). Webmail clients may use either method, but the retrieval protocol is often not a formal standard. Some local mail servers and MUAs are capable of either push or pull mail retrieval.