“On behalf of” Doesn’t Display In Email

Many thanks for any help on this one; I have an administrator who has been a delegate for an executive for quite some time and has always seen the “sent on behalf of” when she sends emails for him. However now, all of a sudden, she doesn’t see it.

She puts his name in the From field and it sends fine, but the recipient sees it as being sent directly by the delegate instead of “on behalf of”. Does anyone know if there’s a setting of some kind that needs to be done (reboot has already been done).