slow smtp/pop transfers

Hello everyone. I currently have a windows 2000 server running the latest version of altn's Mdameon. I am troubleshooting slow smtp and pop transfers from clients that has recently started becoming a problem. Connectivity speed is not an issue. Any traffic besides pop/smtp to this box is 100% fine. There is no traffic shaping anywhere in the network. If more then 1 user connects from the same NATed IP address for large transfers it can time the session out. Internal email works fine. We have tried setting up a new box with new software same issue happens. My ISP and I have noticed with packet captures the server will only send a 8192 byte window requiring an acknowledgement after each window even though the advertised window is 39420. FTP to the same box shows 14 packet window at 35280 and the server sends 2 windows before requiring an acknowledgement. The MTU the Windows sizes in the registry are set to optimum. We have tired new public IP addresses, new server, out side of the firewall, and no anti-virus with no luck. Again all other traffic to this box is 100% fine. All traffic to clients (even using other mail servers) are 100% fine. The network as an average 75-80 round trip time. HELP!!!

Ive also tried using diffrent ports.