[Dovecot] strange connection NG

Dear all,

I have a devocot server based on fedora core 3 in Japan.

When my users are in Japan, there is no any problem reported.Devocot works very well.

But when my users are in China, sometimes the following problem occured.

Sometimes the user can not download mails, after that all connections attempt to server(e.g http/ssh) failed. the user has to close OutLook, and wait for about 10 minutes, then access the webmail to read mail.

Maybe the POP NG is due to the network , but what is the possible reason of 10-minute other connection NG? Is there any solution?

Because users in China share the same outside IP, when one user occures POP NG, all users occures server access NG, that is a big puzzle.

Thanks in advance.

Best regards,