Since your current mail server got an public IP, I assume it's either a DMZ host to the router, or the router is a pass-through device (in which case you usually don't have control over, but check with your ISP to make sure).
You can run the iptables on the current mail server (server A) to redirect the 2525 traffic if:
1) server A has a second NIC connected to the 192.168.0.0 network
2) the mail server under testing (server B) uses server A as its default gateway.
Regarding the gmail/yahoo issue, you have the option at all to change the sending port etc. My assumption was you use email clients such as Outlook/Eudora/ThunderBird etc to test by specifying
xxx.xxx.xxx.xxx:2525 in the outgoing smtp server field.
If you still got more questions, could you post along how your network is set up so I can better (hopefully) answer your questions.