Comment by TheChaplain

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.