dovecot - Sep 2005 - POP3 problem about multi users from one IP address concurrent

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

Additional information

1.when I check the process by 
#ps -ef |grep dovecot

I found about 20 processes with earliest time 8/27 still alive.
Does that matter?

2.PlaceA is 10M broadband, and PlaceB is 512k with about 30 users.
>From: "Yi Xuan" <xuanyi365 at hotmail.com>
>To: dovecot at dovecot.org
>Subject: [Dovecot] POP3 problem about multi users from one IP 
addressconcurrent 
>Date: Sun, 04 Sep 2005 01:46:35 +0000
>
>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
>
>