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SMTP is a protocol used with TCP/IP to send and receive email. It works with POP3 or IMAP, and relays messages across networks. Web-based email users don’t need to worry about SMTP relay, but those with email programs on their devices do. SMTP provides bounce messages for undelivered emails, and began in 1971 on ARPANET.
Simple Mail Transfer Protocol (SMTP) is a set of instructions or procedures in the Transmission Control Protocol/Internet Protocol (TCP/IP) suite used to facilitate the transfer of network data. SMTP is used in conjunction with Post Office Protocol (POP3) or Internet Message Access Protocol (IMAP) to send and receive electronic mail (email) transmissions reliably and efficiently. Simple Mail Transfer Protocol usually handles outgoing mail while Post Office Protocol handles incoming messages. IMAP is a more advanced version of POP3.
When an email is sent, the message goes to the sender’s SMTP mail servers on port 25. A port is an address for transmitted data. The sender’s SMTP server transmits the email message to the recipient’s SMTP server, which then passes the message to the recipient’s local POP3 mail server listening on port 110. This ability to relay email across networks to remote domains is often called SMTP relay.
People using web-based email generally don’t need to worry about the mechanics of SMTP relay because the web server handles it. Users who have email programs on their phones or personal computers typically need to set up their devices to send and receive messages. Web-based email users are usually required to log in to authenticate before they can use their provider’s SMTP servers. This is designed to prevent massive amounts of email or spam from being sent.
Many times web based users use a browser to connect to their email account. The browser communicates via the Internet via Hypertext Transfer Protocol (HTTP) with the web server of the email provider. The provider’s web server then forwards the message to its SMTP server for sending.
Sometimes emails don’t reach their intended destination; in this case, the Simple Mail Transfer Protocol provides a mechanism so that the sender will receive an automatic bounce message indicating that the message was not delivered. There are many reasons why delivery of a message fails. Some reasons could be that the message was identified as spam, the address was misspelled, or the sending or receiving servers were busy. Administrators who send messages using mailing lists containing many recipients may receive many bounce messages. A procedure called Variable Envelope Return Path (VERP) can be used to remove bad email addresses from the list.
The Simple Mail Transfer Protocol began in 1971 and was run on the US government’s Advanced Research Projects Agency Network (ARPANET), which is an early predecessor of the Internet. The ability to send messages electronically is considered by some to be the first major application on the ARPRNET.
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