by 
The Curious Bag
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You may have noticed that your email application on your computer has a settings box for an SMTP server. SMTP is a little-known acronym for a protocol that plays a big role in email service.
SMTP
SMTP stands for Simple Mail Transfer Protocol.
Purpose
SMTP is the primary protocol used to send mail around the Internet.
Servers and Clients
Not only do servers use SMTP to communicate with each other as they relay your emails to their destination, but your email client will use SMTP to send an email to the first server on its journey.
Related Technologies
Related technologies include POP and IMAP. Different email services use these to allow you to retrieve your email from a server. Just remember, POP or IMAP let you check your mail. SMTP lets you send it.
RFC 821
SMTP is a formally defined standard published in the document RFC 821 by the Internet Engineering Task Force. This ensures that multiple applications and companies can be certain that their SMTP software will work with others.
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