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Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission.  SMTP connections secured by SSL, known as SMTPS, default to port 465 (nonstandard, but sometimes used for legacy reasons). Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For receiving messages, client applications usually use either POP3 or IMAP.

Although proprietary systems (such as Microsoft Exchange and IBM Notes) and webmail systems (such as Outlook.com, Gmail and Yahoo! Mail) use their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.

The original design of SMTP had no facility to authenticate senders, or check that servers were authorized to send on their behalf, with the result that email spoofing is possible, and commonly used in email spam and phishing. Occasional proposals are made to modify SMTP extensively or replace it completely.