No idea if this applies to other backends as I don’t have a suitable server for testing. I’m using the latest version of Z-Push from the official repositories on Debian Stretch. If I send an email using an ActiveSync, SpamAssassin on the receiving end adds the following warnings:
This is the message ID, which is indeed missing the domain:
I can confirm that the problem is specific to Z-Push, as when I send an email from the same account on the server, but using Roundcube instead, then the Message-ID header is correctly generated, including the domain.