What Is Simple Mail Transfer Protocol (SMTP)?
SMTP is used to send and receive email. It is sometimes paired with IMAP or POP3 (for example, by a user-level application), which handles the retrieval of messages, while SMTP primarily sends messages to a server for forwarding. SMTP can both send and receive mail, but it's bad at queuing incoming messages, hence the common delegation to other protocols. Proprietary systems like Gmail have their own mail transfer protocols when using their own servers, but they still use good old SMTP to email beyond that.