It would be really useful to have an exhaustive list of all these things that qmail can do that other MTA's can't.

it would be a very small list.

However, the exhaustive list of things other MTAs can do that qmail can't would be huge (starting with backscatter-proof bounces for example)

qmail was limited back in the 90ies and it's horribly behind the times now.

I've got a similar thing with my postfix setup though. My users can go to their account portal and apply for an alias, which when approved (to avoid taking of important names like postmaster etc.) is added to their otherMailboxes LDAP attribute. They can delete the alias themselves when they're done with it.

I'm tempted to start auto-allowing new aliases and just having a list of names that can't be used (postmaster, webmaster) etc, that some services use for domain ownership verification.

As an aside I do hope qmail offers similar limiting abilities. It wouldn't be great if a user could make themselves receive webmaster@domain.tld emails!

They can’t, unless qmail has been configured to know they control mail for domain.tld. The admin configuration mechanism for this (control/virtualdomains) is simple and powerful.

Sorry I'm confused. If your mail server is setup for control of a domain (e.g. domain.tld), then what stops someone from putting postmaster@domain.tld in this .qmail file?

People can put whatever they want in their own .qmail files. That doesn't affect anything unless qmail believes those .qmail files are relevant to what it's delivering. A user controls .qmail files for a domain if and only if an admin has configured qmail to delegate that domain to that user.

Ahh I see. So it's a per domain thing, and you can't restrict more granularly. That's a shame.

I haven't experienced a need for more granular restrictions. Can you give an example of a problem it would solve?

The standard postfix setup these days uses a DB backend and users can edit the database to create new email addresses and aliases.