I work with various clients and some of them have internal email aliases for me. What I usually do is have them automatically forward all my email messages for the internal account to my google email account where it is indexed, searchable and I can reply through there.
Some admins don't know how to set up exchange to automaticlaly forward any internal emails to your external account, so I asked Daniel Petri, who is a god for sysadmins everywhere.
here is his reply, for future reference:
You can do that by creating a contact for your external email (gmail), then, on the user's properties tabs in Active Directory users and computers, go to Exchange General > Mail Delivery, and add that contact as a forwarding address.