example.org and example.com are hosted locally and managed by vmm.

foo@example.org is an alias to foo@example.com, a user account.

I now want to move example.com to another server. However, I cannot
remove the domain because foo@example.com still exists. And I cannot
remove the user because an alias still points to it. So now I have
to remove the alias, remove user, remove domain and then recreate
the alias, which is a bit painful.

Wouldn't a warning do instead?

Thanks for your consideration,