What is SMTP?

SMTP is an acroynm for Simple Mail Transfer Protocol

SMTP is the network protocol used to send emails across the Internet

Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP.

In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.

The usual method of sending mail is your mail client will contact the given SMTP server which looks up the MX record for the destination domain then transports the mail to the destination. If the destination denies this mail it sends a bounce message to the sender, in this case the address in the 'from' field of the mail.