Acronym for Simple Mail Transfer Protocol
It's what email uses for sending.
You can implement secure SMTP using the STARTTLS command.
POP3 and IMAP are what is used for recieving email.
Mail format is specified in 
RFC:822, and then extended with MIME.
Programs which send/recieve SMTP are usually called MTA (Mail Transport Agents)
RFC's:
RFC:3207 - SMTP Service Extension for Secure SMTP over Transport Layer SecurityRFC:3030 - SMTP Service Extensions for Transmission of Large and Binary MIME MessagesRFC:2920 - SMTP Service Extension for Command PipeliningRFC:2852 - Deliver By SMTP Service ExtensionRFC:2645 - ON-DEMAND MAIL RELAY (ODMR) SMTP with Dynamic IP AddressesRFC:2554 - SMTP Service Extension for AuthenticationRFC:2505 - Anti-Spam Recommendations for SMTP MTAsRFC:2487 - SMTP Service Extension for Secure SMTP over TLSRFC:2442 - The Batch SMTP Media TypeRFC:2197 - SMTP Service Extension for Command PipeliningRFC:2034 - SMTP Service Extension for Returning Enhanced Error CodesRFC:1985 - SMTP Service Extension for Remote Message Queue StartingRFC:1891 - SMTP Service Extension for Delivery Status NotificationsRFC:1870 - SMTP Service Extension for Message Size DeclarationRFC:1869 - SMTP Service ExtensionsRFC:1854 - SMTP Service Extension for Command PipeliningRFC:1846 - SMTP 521 Reply CodeRFC:1845 - SMTP Service Extension for Checkpoint/RestartRFC:1830 - SMTP Service Extensions for Transmission of Large and Binary MIME MessagesRFC:1653 - SMTP Service Extension for Message Size DeclarationRFC:1652 - SMTP Service Extension for 8bit-MIMEtransportRFC:1651 - SMTP Service ExtensionsRFC:1428 - Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIMERFC:1427 - SMTP Service Extension for Message Size DeclarationRFC:1426 - SMTP Service Extension for 8bit-MIMEtransportRFC:1425 - SMTP Service ExtensionsRFC:1090 - SMTP on X.25RFC:1047 - Duplicate messages and SMTPRFC:0876 - Survey of SMTP implementationsRFC:0821 - Simple Mail Transfer Protocol30 pages link to SMTP:
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