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IMAP is only one of the many protocols for retrieving email, another one is POP3. These two are the most popular protocols for retrieving email with IMAP being the more powerful of the two. But due to prior support for POP3 in older devices, it is still in widespread use today.
SMTP is undisputedly the most prevalent protocol for sending emails. Although there are other outgoing email protocols, SMTP is the most popular and widely used.
http://www.differencebetween.net/technology/protocols-formats/difference-between-smtp-and-imap/
