it's very simple, my client want to have a gmail account with the sent mail and another with the receive mail (to use them as a backup and friendly search options).
I do this all the time with accounts we need to monitor ....
We just simply toss in an "unseen deliver" line to do that in
the /etc/antivirus.exim filter file for the email address that
we want to monitor and we get copies of all inbound and
outbound mail for that account.