Delayed mail with remote mail exchangers

Hi, I am using CWP DNS + Slave DNS. I have clients using Office 365 email and mail is being delayed by sometimes close to a day. I can only sort out that the server thinks it is responsible for email to those domains.

The only way to set the mail exchanger as remote is to add a dummy email address so I can set the exchanger as remote. Doesn't help and possibly makes the matter worse.

This is crippling me at the moment and I'm running out of ideas. Any suggestions?

Have had to give up on CWP DNS and subscribe to 3rd party DNS. A disappointing turn of events.

MX is handled by DNS.  Simply editing the DNS without doing anything in the mail server itself should suffice.