DA rejects addresses to sites that use Exchange Server

I have a few sites that have separate mail servers such as Exchange Servers.

When I try to send a message to one of those domains the DA mail system won't accept the message and says I'm trying to send to an address that doesn't exist.

Sending from outside my server to their addresses work fine. But if I want to communicate with any of my clients inside a domain that uses an external mail server, I'm screwed.

How do I enable DA to accept my email messages and pass them through to the external mail server?

Other threads found

External Exchange Server

Parked domains to exchange

I haven't solved my problem yet.

Primary domain email on the account is directed to an external mail server
AND
a parked domain email is directed to a different Exchange Server.

I've made changes to the MX records in the Admin section.
When I view the User MX records, it only shows the main domain.

When trying to send from domain on DA server to domain with OFF-DA email server:

"An error occurred while sending mail. The mail server responded: "Unknown User". Please verify that your email address is correct in your mail preferences and try again."

Click to expand...

PLEASE MOVE THIS THREAD to DNS forum

I apologize for posting this in the wrong place. Please move to DNS or Email forum.

Hey,

This is one of those posts where it would be real handy to know the domain name you are having problems with...

David