Support SMTP AUTH for submission use #7

Summary

Add SMTP authentication (e.g., PLAIN/LOGIN) so authenticated clients can submit mail without relying solely on allow-listed networks.

Motivation

Today the server rejects connections unless they originate from explicitly allowed hosts or networks, which limits deployment in semi-trusted environments. Providing SMTP AUTH would enable secure submission from users who aren’t on the allow-list and integrate better with upstream MTAs.

Proposed direction

• Implement SASL PLAIN/LOGIN (minimum) using environment-configured credentials or pluggable backends.
• Ensure AUTH is only offered when TLS is in use (or explicitly permitted) to avoid credential leakage.
• Document configuration and potential security considerations.

Labels

• feature
• authentication