Some times when we delete user mailbox, and the user email still exist in other email accounts as (send on behalf) for example, then if we try to for example to grant (send on behalf permission) for new user, we might will receive error like the following:  


My Solution:
go to      AD Users & Groups-----> then select email account having problem----->properties---->
select Attribute Editor------>select PUBLIC DELEGATES----->then remove the error mailbox account.

this should solve the problem.

Enjoy