Email Account Doe Not Exist

The following recepient(s) could not be reached:
Usreb and Date
The e-mail account does not exist at the organization this message was set to.
USERB has been delegate on USERA mailbox.

In Active directory,I check USERA account >Go to Exchange general > Delivery options > Send on behalf >Removed USERB from the list. But There’s is no user to remove. Userb was removed from the Network months ago. Why do we get it and how do I eliminate it?