I have in the postfix conf:
relayhost = my.internal.relay.example.org
All the mails transit through this relay. by default, postfix will try to do a MX lookup on my.internal.relay.example.org
It's not the purpose of MX record. I just want to send through this host, not to the domain.
I know how to fix it (with  or postfix config). postfix manuals says how to change the behaviour but not the reason.