#550 4.4.7 QUEUE.Expired; message expired ##

I have a client who is getting emails not delivered from the Department of defense. The email had an attchment that got the #550 4.4.7 QUEUE.Expired; message expired ## erro. Had her remove the attchment and send to the same people with return receipt.

Both people got the email but return reciept never came back. I am having a hardtime with this issue.

Thanks