INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"Unknown User"

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"Unknown User"

Hi,
I am on a subscription plan since couple of months. Many  of our users are facing this peculiar problem.
They are unable to reply to an email they have received. When they reply they get this "INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"Unknown User" and the mail bounces.
Any idea why?
I searched the forum/FAQ and I could not find anything relevant.

Let me know, if you need more details.I can forward the complete message.

Thanks
Ravi