Simple Mail Transfer Protocol (SMTP) is a standard for e-mail transmissions across the Internet. SMTP is a relatively simple, text-based protocol, in which one or more recipients of a message are specified (and in most cases verified to exist) along with the message text and possibly other encoded objects.
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E-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection.
The IMAP (Internet Message Access Protocol) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on a remote server.