SMTP is an Internet standard for the transfer of email. The name is short for "simple mail transfer protocol," which describes its simplicity and text-based nature. It is used by email servers to both send and receive email. However, email users will be more familiar with it as a sending protocol, as that is its only common use on the user level.
An SMTP email server is used when an individual wants to send email through a client such as Microsoft Outlook, Mozilla Thunderbird, and others. Some web-based email clients, or webmail, allow individuals to add external email accounts and set them up to send, receive, or do both. Gmail from Google is one such service.
In cases where the user needs to set up the account in order to send email, then providing the SMTP address will be required. Some common addresses are smtp.domain.com or mail.domain.com, although it can vary significantly depending upon the service the domain name is using. Where the email server is on an outside domain, the address may not even include the domain name that the email uses, for instance: smtp.outsidemailserver.com.
The SMTP server, its address, and the emails to be sent through it, are sometimes referred to as outgoing mail. Normally found by looking up the MX DNS (mail exchange domain name server) on a domain name, the relay server attempts to find the proper server to which to connect. If no MX is found, some relaying servers will then use the fall-back option of the A record type. By default, SMTP uses a TCP connection on port 25 on the server, although this may be changed with configuration.
Some Internet service providers (ISPs) have come to block port 25, in an attempt to stop their services from being used by people who send spam or excessive emails. When this is done, the ISP typically only allows port 25 to be used in conjunction with email provided by its own service, but not other SMTP servers, such as the user's own domain name, or that of a third party.
Since SMTP is used by most individuals only for sending email, they still need a method of receiving incoming mail. This is normally done through post office protocol (POP, usually POP3 for version 3) or Internet message access protocol (IMAP). Extended turn (ETRN) can also be used by SMTP to request another mail server to forward emails to it.
Local mail transfer protocol, or LMTP for short, is a derivative of SMTP, and meant to be an alternative for it in cases where there is no mail queue available on the receiving end. Using a mail deliver agent email storage server is one such situation. Additionally, LMTP has the capability of noting success or failure for each address in a case where a message has multiple recipients, whereas SMTP can only state it on an "all or nothing" basis.