If you have any domains that does not use email, it may be a good idea to set up some DNS records to prevent it being used.

DNS SPF record: mydomain.io. TXT "v=spf1 -all"

DNS DMARC: _dmarc.mydomain.io. TXT "v=DMARC1; p=reject; sp=reject; adkim=s; aspf=s"

That ought to stop anyone trying to use your domains as source.

Yes! IMHO every registrar should be turning this on by default. Every DNS should do this by default until the owner explicitly turns on email sending.

It would solve a lot of issues globally.

Also consider (using your example domain):

  *.mydomain.io. TXT "v=spf1 -all"

I use postfix and the recipient_access file to control email to my domains which use little email, so the domains are able to process standard email:

admin@example.com OK postmaster@example.com OK abuse@example.com OK webmaster@example.com OK hostmaster@example.com OK info@example.com OK example.com REJECT example.com