SMTP (Simple Mail Transfer Protocol)
is used for sending and receiving e-mail. However, as it is “simple,”
it is limited in its ability to queue messages at the receiving end,
and is usually used with one of two other protocols, POP3 or IMAP.
These other protocols allow the user to save messages in a server
mailbox and download them periodically from the server.
authentication is an extension of SMTP, whereby the client is required
to log into the mail server before or during the sending of email. It
can be used to allow legitimate users to send email while denying the
service to unauthorized users, such as spammers.