Currently the way to do this would be to write a custom_check_script which interacts with the virustotal api:
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmgconfig_custom_check
The idea sounds good in general - but as far as I could find out - directly integrating it in PMG could be problematic:
https://developers.virustotal.com/reference/public-vs-premium-api
* the free API is explicitly not for commercial products and services (so we at least would need to make this very explicit for the users)
* the rate-limits are a bit too restrictive for a mail-gateway (4/minute, 500/day)
I would be curious how the premium API is priced (could not find anything on a quick search) - if it's somewhat reasonable, I could imagine implementing some kind of integration like PMG has with avast.
So should you have a premium API account or have contacted them - I would be grateful if you share your experiences