Port Monitor

I'm pretty sure something is sending spam through my Exchange server.. does anyone know of a windows tool that would help me see what processes are using port 25? I'm looking around Sysinternals but having a hard time figuring out if my solution is there.

Edit: It's not relaying for outside servers..it's just sending off a ton of data. I can't tell where at this point though I'm beginning to think it's a misconfiguration with my ISPs switch.