Using Identities to send Emails, question on Email Storage

hi, i was wondering that when sending mail with a different identity(bob@mydomain.com using help@mydomain.com identity), why does it not copy the message to help@mydomain.com(Sent box). instead it only shows it on bob@mydomain.com(Sent box). appreciate if someone could advise.

could someone advise how to fix this as i would like to see the message in help@mydomain.com(SentBox) too.

thanks kevin