Caught socket exception in SMTP/POP

[AlanYuan]

We have an email server in our head office, without any firewall or router obstructed, using real IP address directly connect to internet。Each mail come from foreign is ok and some colleagues can receive their mail at home this couple years。

Recently our factory using Wingate as the mail proxy, they get problems while receiving huge mail or sometimes cannot connect to our mailserver. Usually everything fine in morning after they power on their server, but go worst gradually, a few hours later even they can't access our webmail (same IP address, port 32000), however they can ping our IP address and our domain name while the mail is in trouble。

Also their Wingate has another smtp/pop connected to another ISP, that means they have two smtp/pop ports assigned。When our train is in trouble, the another is still good。The log is show below :
Error: Caught socket exception in CTCPMappingSession::OnRead() Connection Reset by Remote Host - terminating
Error: Caught socket exception in CTCPMappingSession::OnClose() Connection Reset by Remote Host - terminating
Error: Caught socket exception in CTCPMappingSession::Initialize() Connection Reset by Remote Host - terminating
Error: Caught socket exception in CTCPMappingSession::Initialize() Host name lookup for 'mail.knt.com.hk' failed - terminating

Previously they can restart the smtp/pop service and then everything back to normal, but now this action has no effect。
The debug log is below :
08/24/05 15:47:49 77.8.16.20 Guest 0000031073 Created:
08/24/05 15:47:49 77.8.16.20 Guest 0000031073 Requested: TCPMapping: Connected to 210.3.11.3:25
08/24/05 15:47:54 77.8.16.20 Guest 0000031073 Traffic 609 13857 13857 609 6s
08/24/05 15:47:54 77.8.16.20 Guest 0000031073 Terminated exit code 0

Also, about one month ago, we had lost the connection about 30 minutes, but without any action in both side, the mail back to normal again.

Pls help me how to troubleshoot the problem, Thanks !