Email system architecture

A user typically composes mail in a mail user agent (MUA). The message is submitted to a mail submission service, transferred between mail transfer agents (MTAs), filtered by gateways or security services, and ultimately stored for the recipient.

The useful mental model

Separate submission, transfer, filtering, and mailbox access. SMTP handles submission and transfer; IMAP and POP3 are mailbox-access protocols. Hosted platforms may hide these boundaries, but the boundaries still matter when troubleshooting.