What if starting a PM via email was accepted only from email addresses corresponding to a registerd user (of some TL)? If the sole reason to not allow this is because of possible spam would that solve it? How hard is it to impersonate a “from” email. Also noted is that this would be an admin’s choice to allow this. Like the OP I run a small closed registration instance. Not a lot of chance some bot would randomly know both a sender email address in our instance and addressie username in instance.
If there are coding effort/reasons well, that’s another thing.
Either way likely not to be added to the core but could be at least initially a plugin?