there is such a thing as machine- or even application-specific accounts that can be terminated or their credentials rotated.

Even better; you could use something like Vault (https://www.vaultproject.io/) to manage such application-specific credentials.

All of this is much better than relying on IP addresses never changing and never being reused.

>Why is it when Debian patches postfix that it's still (in your mind) "postfix proper" but when I download a well-distributed and discussed patch from a mailing list, and read it myself, that you somehow think I'm at higher risk?

I never said it was postfix proper. It really isn't.

Debian is known to f'up the security of its packages. But it's my hope that distributors have a better equipped security team and bigger exposure compared to some guy putting a patch on some website.

Debian also has a proper security update process that makes sure that patches go out to users quickly and painlessly. And because Debian is big enough, it's usually included in embargoed security vulnerability reports, so usually a patch will be out and sometimes even deployed by the time the vulnerability is known.

But the relative security of Debian-maintained postfix vs. Debian-maintained custom-patched qmail is the same as postfix proper vs custom-patched qmail.

However, vanilla whatever, in my opinion is superior to custom-patched whatever. And while postfix is usable vanilla, qmail isn't. It was bordering unusable back in the 90ies and it definitely is unusable nowadays.

qmail doesn't support TLS, it doesn't support spf, much less domainkeys or dmarc. It doesn't (as we're talking about) support SMTP auth, it doesn't support ipv6. It doesn't support any other common anti spam measures like greylisting or rbl checks, nor does it support a plugin system that would allow for easy plugging them in.

Heck, it doesn't even support rejecting mail at SMTP time so it's one of the worst offenders for backscatter which is one of the big annoyances for mail admins.

At this point, an unpatched qmail is as useful in fulfilling MTA roles as a closed port 25 is (which would totally be safer than both qmail and postfix)

Hence my assertion that everybody is running a patched qmail and none of these patches ever enjoyed the same scrutiny as even the security abominations that are sendmail and exim.

And worse: None of these patches were ever reviewed for side-effects of arbitrarily combining them.

Nearly every custom patch was made against a vanilla qmail because there is no central project to manage these patches. So every site runs a different version of qmail, none of them vetted at all for security.

This is a mess and we should really discourage this rather than dreaming of long-gone times where vanilla qmail at least was useful to a minor degree.

Partially-related update: I just looked at the qmail code: This thing is written in K&R style C and sometimes uses single-character variable and function names. I would argue nobody but djb can actually safely make changes to this code-base. This is all from a bygone age. Let it rest.