SMTP, an Internet Standard for Electronic Mail Transmission
SMTP stands for Simple Mail Transfer Protocol that is a standard protocol for email services on networks that use TCP/IP. It is also commonly known as RFC 2821 and RFC 821.
SMTP is one of the most wide-spread and popular protocols used for email communication over the Internet. SMTP is used to provide intermediary network services between the organizational email servers or remote email providers and the local users accessing it.
SMTP is usually built-in within an email client application and consists of the following essential components:
- Server which is known as mail submission agent (MSA)
- Client-end or local user utility which is called the mail user agent (MUA)
- Mail delivery agent (MDA)
- Mail transfer agent (MTA)
SMTP operates by starting a session between a local user and the server while MDA and MTA ensure such services as local delivery and domain searching. As SMTP features a limited ability to queue messages at the receiving end so it's typically used with one of two other protocols, IMAP or POP. These protocols allow users to save email messages in a server mailbox and download them regularly from the server. Users commonly use a special program that utilizes SMTP for sending email and one of the two protocols – IMAP or POP3 for receiving email.
Although webmail systems like Yahoo and Gmail and proprietary systems such as IBM Notes and Microsoft Exchange use their own specific protocols to get access to mail box accounts on their proprietary mail servers, they all use SMTP when receiving or sending email from users outside their own systems.
SMTP is usually enabled to operate over Internet port 25. There is a European alternative to SMTP – X.400. These days, a lot of mail servers provide support of Extended Simple Mail Transfer Protocol (ESMTP) that makes it possible to deliver multimedia files via email.