blind copy transport rule

I want to set up a transport rule that blind copies all internal emails for all users to an external mail system. The email address for the external system is based on the AD samaccountname for each user.

So if the user has mail address   forename.surname@example.com   and their AD account is   fred

Then I want a BCC sent to       fred@external.example.com

I know I can create individual rules for each user but I figure there must be an easier way.