I haven't tried it, honestly, but I'd imagine it'd work fine. The SQL usage there isn't very complex, so I'm pretty sure the syntax would even be exactly the same between the two for the few things I'm using it for. There is some difference in the connection strings though. It looks like Gentoo has some good docs on this in their Wiki at https://wiki.gentoo.org/wiki/Complete_Virtual_Mail_Server/Po...

Good luck, let me know how it goes, and I'm happy to accept a pull request to add additional instructions for Postgres if you feel up to adding them.