SMTP stands for: Simple Mail Transfer Protocol. It is a protocol that is used by a server for sending email.
What is a SMTP server
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What is an outgoing mail server?
A SMTP server is an outgoing mail server - a server that sends email to another server: the receiving server.
If you send an email to someone, as soon as you press the "Send" button, your email program will communicate with the SMTP server
that you have configured in your email software settings. It will then send the email to the SMTP server.
The SMTP server in turn, will try to locate the receiving (POP3 or IMAP, or incoming mail server), of the person you want to send the email to. The SMTP server will then send your email to the receiving / incoming mail server that is used by the receiver of your email. The
incoming mail server in turn, will simply receive the email and save it. Next time the receiving person opens
his email program, he will be notified of new email and receive your email.
What is a POP3 or IMAP server
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What is an incoming mail server?
A POP3 or IMAP server is an incoming mail server. It doesn't send email, it only receives emails from other servers and saves them for its clients.
As soon as somebody opens his or her email program, it will notify that person of new email.
Often the SMTP and POP3 / IMAP servers are the same server, performing both tasks. To address the right protocol, they have names like "smtp.domain.com" and "pop.domain.com".
The difference between IMAP and POP3 is simple: An IMAP server will always keep the emails on its hard drive; the person downloading emails, is simply downloading a copy of it, but emails are kept on the server unless the person explicitly tells the server to delete them. POP3 on the other hand, will delete the emails as soon as the receiving email program downloads new emails.