not sending to local server

Hi - I’ve been running a GPL server which has been working very well for about a year now (until this problem). The issue is that I have one domain on my server, where postfix sends to my relayhost instead of delivering a message locally (she is sending to another local domain). Sadly, my relay server isn’t working right and not routing it correctly so it is bounced back to the user. While I could modify the relay host, I’d rather not as that presents problems outside the scope of this question.

However, if this one domain would deliver it locally instead of sending out, all would be well. Any idea what to check or what’s going on? Other domains on my server are working correctly…