Можно попробовать создать правило на сервере, запрещающее автоматически (я так понимаю, что речь идет именно об автоматическом перенаправлении) форвардить наружу, если есть поле MailFwdBy.
Forwarded messages that were forwarded manually by a user contains a field called ForwardedFrom in the Fields tab of the document properties. Documents that were automatically forwarded by this mail rule contain a different field, MailFwdBy, which is exclusive to mail that is forwarded using a rule. If you get a message containing this field, it was forwarded by a rule.