Having email forwarded into me and changing reply to options

[Hal]

Hello,

I am having an outside email address that sits on an Exchange Server
forwarded to my normal email address. I want to figure out a way that when I
receive it, I can reply with the Exchange Server email Address, rather than
the address that it is being forwared to.

Notes: I am not replying back to the exchange server, rather through a
different conection. I cannot pop or connect to the exchange server directly.

Thank you!