Setup a second MX record for testdomain, and have it point to your current IP address.
That won’t help. The router routes port 25 to the Internal IP address for mydomain.com. I can’t route port 25 to two different internal IP’s.
In Exchange System Manager, create a second SMTP Virtual Server and set that to port 2525. Then port forward that port.
That sounds like a logical solution. However, right now I port forward port 25 to mydomain internal server IP address, I can’t port forward to 2 different IP addresses.
Port 2525 or your choice on your second server. Leave port 25 for your original server.
AsI thought, Making a new virtual smtp server on port 2525 and forwarding my router port 2525 to testdomain did not work. By default all smtp incoming traffic uses port 25!