There is an argument to be made for some sort of central backend store when you have multiple mail servers for the same domain(s) and users. SQL isn't terrible

There is an argument to be made for some sort of central backend store when you have multiple mail servers for the same domain(s) and users. SQL isn't terrible in that context (I would likely prefer LDAP for that, but SQL works, too). Most of our users have one server, and lots of tutorials and stuff are about one mail server. It just makes no sense to add that huge amount of extra complexity for that simple use case.