Having just trawled through this thread I have an idea that I've not seen mentioned so far — did anyone in
VPS Hosting Support look at the /etc/antivirus.exim file (which I presume may well be installed on a
VPS Hosting), because until a few days ago this was unknowingly discarding incoming and outgoing mail on my
dedicated server (as it was pre-configured to reject many spam-related keywords/phrases 'out-of-the-box', as it were). I was lucky to spot what it was doing when I was studying the Exim_mainlog ...
Sorry if this is a red herring...!