It would certainly be possible to support mail-receiver on ARM systems. There’s nothing amd64-specific in there (or, at least, there wasn’t when I made it, and I can’t imagine any major changes would have been made to invalidate that assumption). It would just be a matter of the docker image maintainers making a build of the image for arm64, as they already do for amd64.
Someone could also do an unofficial build and put it somewhere, along with instructions on the one-line change needed to the pups template, or you could do your own local build from the repo on the arm64 system you’re running on.