If you want to create a Rule that will
email a copy of all messages you send to an external email address, then
open up your mail file, go to Tools-Rules and create a new Rule:
Sender - Is - You
Send copy to - <the external email addresses>
For existing messages in your Sent folder,
an agent/actionButton can be written that will allow you to select messages
in your Sent folder and email those to an external mail file (if you are
in an environment set up mail externally). Reply if you need help
with this script.
There is also the 'Queryaddtofolder'
event in the Sent folder that could be programmed to send a copy of any
message added to to folder to an external email address.