Description

This was primarily to address the final section of this SMTP_ONLY=1 comment regarding a logged error:

postfix/submission/smtpd[906]: error: open database /etc/postfix/vmailbox: No such file or directory

• This was due to main.cf being configured for Dovecot by default, instead of updating relevant config during startup. When SMTP_ONLY=1 this file does not exist as it's not relevant.
• Rather than take a subtractive approach to clear it out, I've opted for moving that into the startup scripts.
• As the virtual transport is related, I moved that as well, and co-located the similar ENV feature for overriding the transport (which I'm inclined to deprecate in favor of user-patches.sh).

Unrelated changes have been bundled:

• main.cf comments to clarify the source of files references / generated.
• Startup scope better clarifying the dependency and a TODO regarding a concern for /etc/postfix/regexp being always created.
  • That requirement context was added in FIX: Postfix configuration in start-mailserver.sh for regex #1754 that also introduced the unionmap feature that will need some revision before SPOOF_PROTECTION=1 can be default.
• Additional comments to SASL Auth logic, with TODO that the approach should be flipped.
  • Reference issue where no SASL auth was expected: [BUG] SASL is enabled with ENABLE_SASLAUTHD=0 but only when SMTP_ONLY=1 #1796
  • ENABLE_SASLAUTHD=1 || SMTP_ONLY -ne 1 should enable the feature on submission(s) services (instead via postconf not sed)?
  • This is from a features based approach, instead of defaults focused.
    • It's not as nice from a source view but it should better avoid such changes being missed (as was the case with config: disable SMTP authentication on port 25 #3006 ).
    • Similarly, in the past the original logic for this (to address "fatal: no SASL authentication mechanisms" when running SMTP_ONLY=1 #526 ) was missing the master.cf fix.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md