Sorry to be a pain, I am trying to rephrase an earlier question I wrote (badly, so don't want to link it)!

If I am running internal e-mail, can I specify a filter so that any e-mail which attempts to go externally (routed via an external SMTP provider), the e-mail address gets changed from it's internal address to some master external e-mail address?

Basically, joe.bloggs and henry.thacker can converse internally, but if one decides to send a message "outside", their FROM address should both be replaced to: sales@mydomain.com. I only need to set this one replacement up.. would this be straightforward?