What does RFC 5321 say about MAIL FROM and HELO names?

RFC 5321 specifies requirements for MAIL FROM and HELO/EHLO identification during SMTP sessions. The MAIL FROM address should be a valid, monitored address that can receive bounces. The HELO/EHLO name should identify the sending server with a valid, resolvable hostname.

The RFC requires HELO to present a fully qualified domain name matching the sending server's identity. Presenting IP literals or unresolvable names violates the specification and triggers filtering at many providers.

Proper MAIL FROM and HELO identification enables bounce handling and accountability. Deceptive or broken identification breaks email delivery mechanisms and signals illegitimate sending to receivers evaluating compliance.