Mail to Requester - Sending notification failed

Mail to Requester - Sending notification failed

We have two domains within our organization. When we try to reply to a requester in the alternate domain through the helpdesk the email address will populate the field but then we get a window that says: Sending notification failed.

Apparently we can not send email to any addresses outside of our main domain. This used to work and now it doesn't.

Please advise.
thanks.