Replacing Sendmail on the SMTP-OUT Tier

Background

With the containerization migration (see docs/0.4.x/containerization-plan.md), Dovecot now handles user-facing SMTP submission on the smtp-out tier (ports 587/465), authenticating users via PAM against Cognito. Dovecot then relays to sendmail on localhost:25 for outbound delivery. This architecture reduces sendmail’s role significantly, making it a good candidate for replacement.

What Sendmail Currently Does on SMTP-OUT

Sendmail is not a dumb relay. It performs:

Masquerading — Rewrites From: addresses using a dynamically-generated masq-domains file (all hosted domains from DynamoDB).
Mailertable routing — If the recipient is on a hosted domain, routes the message back to the IMAP container (smtp:[imap.internal]) rather than sending it externally.
DKIM signing — Passes outbound mail through the OpenDKIM milter.
Access control / rate limiting — 500 messages/day/sender, 100 recipients/transaction.

The greet pause that was previously configured has been removed, as the only client connecting to sendmail on smtp-out is Dovecot on localhost.

Recommended Replacement: Postfix

Postfix is the most straightforward replacement. The configuration mapping is nearly 1:1:

Sendmail feature	Postfix equivalent
`masq-domains`	`masquerade_domains`
`mailertable`	`transport_maps`
`access` db	`smtpd_client_restrictions` / `check_client_access`
`INPUT_MAIL_FILTER(opendkim)`	`smtpd_milters` / `non_smtpd_milters`
`makemap hash`	`postmap hash`

Migration effort

Rewrite out-sendmail.mc as Postfix main.cf / master.cf.
Update generate-config.sh and reconfigure.sh to use postmap instead of makemap and to signal Postfix (postfix reload) instead of sendmail.
Update the Dockerfile to install postfix and postfix-pcre instead of sendmail and sendmail-milter.
The DynamoDB query logic in the config generation scripts stays the same.

Why not OpenSMTPD?

OpenSMTPD has a simpler configuration language but weaker milter support. Since we rely on the OpenDKIM milter for DKIM signing, Postfix’s mature milter integration is the safer choice.

What About SMTP-IN?

Sendmail on smtp-in has a more complex role (inbound delivery, alias resolution, virtual user handling) and benefits from the greet pause for anti-spam. Replacing sendmail on smtp-in is a larger effort and should be evaluated separately.