PMG is (currently) amd64 only (and we're not having any active plans on providing the packages for other architecture at the moment)...
things that are binary and won't work without recompilation (from the top of my head - so it's only a rough guess):
* kernel (use the proxmox-mailgateway-container meta-package then the kernel is not pulled in)
* pmg-log-tracker (a rust-binary)
* I think SpamAssassin also might have some architecture specific code inside
I hope this helps!