[Dovecot] Dovecot/postfix to do 'copy to sent' ?

I always keep a copy of messages sent.

With a particularly large message (pictures etc), being sent from a far remote location, I noticed that the message and the copy were both sent from the client - rather than having the message sent (through my mail server) and then copied locally within the mail server.

Is there a way to configure Dovecot (or postfix?) so the local copy is done?

Thanks for your time.

Bob G