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.
How Does SMTP Work?
SMTP is an asymmetrical protocol, meaning that there are many clients interacting with one server, using a basic model popular in the 1980s which is now mostly defunct outside of email protocols. SMTP runs on TCP/IP and listens on port 25.
What's the Difference Between SMTP and Extended SMTP?
ESMTP is a new version of SMTP with added functionality. Released in 1995, it can send multimedia files. Now your aunt can attach videos of her cat to her weekly email update. Thanks, Internet.