A client sends EHLO and the server returns the ESMTP capabilities available for that connection.

Common examples include:

  • STARTTLS
  • SIZE
  • PIPELINING
  • 8BITMIME
  • SMTPUTF8
  • AUTH
  • DSN

Capabilities can change after STARTTLS or authentication, so clients should follow the protocol rather than assuming a server always advertises the same list.

Reference material