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Simple Mail Transfer Protocol (SMTP)

Internet standard. The communication rules that systems follow for the reliable and efficient transfer of email messages. RFC 821(link takes you to an external page) defines this standard.

SMTP workflow

smtp-workflow page anchor
  1. You compose and email and press Send.
  2. Your Mail User Agent (MUA), the email app, adds some metadata called headers to your email message. These include To, From, and Subject.
  3. The message passes to a Mail Transfer Agent (MTA), usually an SMTP server.
  4. The MTA performs a few operations:
    1. Decodes your email.
    2. Determines which server the message must be sent to for your recipient to retrieve it.
    3. Adds routing metadata like Received, Date, and other data.
    4. Issues SMTP commands that start the routing the email message to the receiving server.
  5. If the email message crosses domains, the messages use an SMTP relay.
  6. When the recipient's email provider, like Yahoo! or Gmail, receives the message, it downloads and delivers it to the appropriate inbox.