postfix service: don't empty local_recipient_maps

[abbradar]

Motivation for this change

From Postfix documentation:

With this setting, the Postfix SMTP server will not reject mail with "User
unknown in local recipient table". Don't do this on systems that receive mail
directly from the Internet. With today's worms and viruses, Postfix will become
a backscatter source: it accepts mail for non-existent recipients and then
tries to return that mail as "undeliverable" to the often forged sender
address.

However, I'm unsure that this is a right fix to do for everyone. Postfix can be configured in a multitude of ways: usually on my servers I set local_recipient_maps to a table with users, but perhaps for some other configurations this is not necessary.

Also this is a breaking change, but one that gives us more secure defaults.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@abbradar, thanks for your PR! By analyzing the history of the files in this pull request, we identified @uwap, @edolstra and @peti to be potential reviewers.

[peti]

IMHO, it's perfectly okay to merge that change. Most mail servers use virtual aliases for their users anyway, i.e. the host's local name is typically not equal to the domain name, and in that case this change in behavior will not affect people anyway. That's the case on my server, at least.

I don't think we should back-port the change to release-16.09 though.

I would like see an entry added to nixos/doc/manual/release-notes/rl-1703.xml, however, to make sure users updating from one release to the next are aware of this modification.

[abbradar]

I agree that this shouldn't be backported as this is a breaking change. Added an entry to the changelog.

[abbradar]

Thanks to @peti and @fpletz for review!