[Dovecot] POP3 problem about multi users from one IP address concurrent

[Yi Xuan]

Hi,all

I have a POP3 problem about multi users from one IP address concurrent.

[Issue description]
The network architecture is as following.
I have setup a mail server(sendmail/dovecot) in PlaceA with IP_A.
And about 6 users are receving mail from the same IP_B( NATed public IP) in 
PlaceB.
Their mails are many( about 1Mb/hour each) and ususally set fetch time as 
10min/fetch.

Then they complain that their mails often cannot be downloaded. 

[Reason investigation]
I noticed that 
1.when one of them fail to fetch mail. All TCP IP connection between IP_A 
and IP_B will be RST.which means that http/ssh/pop/smtp all not work for a 
while.
And after about 10 minute, the http/ssh can work well.
2.I have asked them to backup and delete pop accounts and use webmail 
through imap provided by dovecot.And that seems work well but inefficient.

I think this maybe because of multi users from one IP address concurrent,

[Question]
Would you pls provide some suggestion or solution about this issue?
Is this a bug of dovecot or just a mis-configuration I don`t know?

Thanks in advance for any advice.

Best regards,
Yi Xuan