Weird, no mail server but forward works

So I have a bad boy client......my brother....ugg.   He got me in trouble a few years ago when he started spamming people.  So I kicked him off my server.   Now fast forward 2 years and I decided to give him a another chance.  But this time, I set his email accounts to 0 by creating a custom package just for him.  Great, that worked.   But he was able to set up a forwarder from his domain email.  Uggg.......

So he can advertise his personal email on his websites and other nefarious locations.  Then his emails will be forwarded to his Gmail account.

I think this is a bad situation.

that only need a MX rule.. lock the dns setting

in fact you cant do that too...
he only need point IN NS to another place and IN A to your server

typo.. lock = look

that only need a MX rule.. lock the dns setting

in fact you cant do that too...
he only need point IN NS to another place and IN A to your server

I control his dns.