With webfinger, you should be able to do exactly that. Migration of accounts is not in E-Mail, but while email has mx servers, Matrix (and most nu-fed stuff, not sure about XMPP) should have webfinger support for that.

edit: Okay, not actually webfinger. But [0] has instructions for the `/well-known/matrix/server` way. It only talks about subdomains, but it should work across domains. Possibly also with the SRV header.

[0]: https://github.com/spantaleev/matrix-docker-ansible-deploy/b...