Capture SMTP forward different host

[deresistance]

I am not sure how to go about this.  Basically a user behind PFSense on the internal network attempts to send an email to an SMTP host example mail.server.com  but I want PFSense to capture anything going out port 25 rewrite the destination server to myrelay.mydomain.com so that it goes out a different relay.    (Not worried about outbound authentication I know that will kill it)

I was looking at the postfix relay package for PFSense not sure if you can do it in the GUi or if I could custom a config file to do this.    Any ideas would be great.  Thanks in advance