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 network data transfer. SMTP is used along with Post Office Protocol (POP3) or Internet Message Access Protocol (IMAP) to reliably and efficiently send and receive electronic mail (e-mail) transmissions. The Simple Mail Transfer Protocol usually handles outgoing e-mail while the Post Office Protocol handles incoming messages. IMAP is a more advanced version of POP3.
When e-mail 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 e-mail message to the receiver’s SMTP server, which then hands the message off to the receiver’s local POP3 mail server listening on Port 110. This ability to forward e-mail across networks to remote domains is often called SMTP relaying.
People who use web-based e-mail generally do not have to worry about the mechanics of SMTP relaying because the web server handles it. Users who have e-mail programs on their phones or personal computers generally have to configure their devices to send and receive messages. Web-based e-mail users are usually required to login to authenticate themselves before they are allowed to use their provider’s SMTP servers. This is designed to prevent the sending of massive amounts of e-mail messages or spam.
Many times web-based users utilize a browser to connect to their e-mail account. The browser communicates via the Internet using Hypertext Transfer Protocol (HTTP) with the e-mail provider’s web server. The provider’s web server then relays the message to its SMTP server to be sent.
Sometimes e-mail messages fail to reach their intended destination; in this case, the Simple Mail Transfer Protocol provides a mechanism so the sender will receive an automated bounce message that says their message was undeliverable. There are many reasons for a message delivery to fail. Some reasons could be that the message was identified as spam, the address was misspelled, or the sending or receiving servers were busy. Administrators that 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 e-mail addresses from the list.