BarokOnline

At first, I got my complicated minds thinking of broader solutions but here's a simpler one.

You can forward your existing Exchange email account to external email domain. You must have an administrator account to be able to do this on your Exchange Server.


1. Create a contact in your Active Directory pointing to the external email.
2. On the user's Exchange account, set forwarding to the contact you just created and you're done.
Simple as that.