SMTP (Simple Mail Transfer Protocol)

A TCP/IP protocol used in sending and receiving e-mail.

Communication between mail servers uses TCP port 25 (tranditional) and 587 (recently)

SMTPS: SMTP connections secured by TLS

Transfet Model

• MUA (mail user agent):
  • mail client
• MSA (mail submission agent):
  • receives mail messages from a MUA from TCP port 25 or 587
  • delivers the mail to its MTA
• MTA (mail transfer agent)
  • uses the DNS to look up the MX record (mail exchanger record) for the recipient's domain (the part of the email address on the right of @).
  • final MTA server hands the message to a MDA for local delivery.
• MDA (mail delivery agent):
  • delivery of e-mail messages to a local recipient's mailbox.
  • Also called an LDA, or local delivery agent

Mail is retrieved by end-user applications, called email clients, using IMAP or POP

• IMAP (Internet Message Access Protocol):
  • always syncs: any changes you make in your mail client will instantly appear on your webmail inbox.
• POP (Post Office Protocol):
  • not synced: whatever changes you make to your email account in the mail client will not be transferred to the webmail inbox.
  • typically uses the traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook