What is SMTP AUTH and why is it important?

SMTP AUTH (SMTP Authentication) requires senders to prove their identity before a mail server accepts messages for delivery. It's a critical security mechanism preventing unauthorized use of mail infrastructure.

How it works:

Client connects to mail server on port 587 (submission) or 465 (SMTPS)

Server advertises AUTH capability during EHLO handshake

Client provides credentials (username/password, OAuth token, or certificate)

Server validates credentials before accepting messages

Common mechanisms: PLAIN, LOGIN (over TLS), CRAM-MD5, XOAUTH2

Why it matters:

Early email had no authentication. Any server could relay mail from anyone. This enabled spam and abuse. Open relays that accepted unauthenticated mail became vectors for mass spam.

SMTP AUTH ensures only authorized users can send through a server. It enables:

Accountability (every message traces to an authenticated sender)

Access control (only valid accounts can send)

Abuse prevention (compromise requires credentials, not just connectivity)

Reputation protection (sending limited to authorized users)

Modern ESPs and email servers require SMTP AUTH. Unauthenticated submission ports (25) are typically restricted to server-to-server communication, not client submission.

It's checking credentials before letting anyone load cargo onto the ship.