Applescript to forward email but preserve sender
You should be able to use the "Redirect" option, under the Message menu. Actually, if I'm reading your question right, you should be able to do this entirely through a rule, with no Applescript needed. 

Building from "How to batch forward messages from Mail.app", if you make a rule that targets emails sent to mac@email.com, just say "Redirect Message" to "email@gmail.com".

Example: 

Make sense?