This is a complicated problem. The configuration is
Internet Connection: non-static IP, HSDPA 3.5G Card, or ADSL.
client: Zimbra Desktop 7, or outlook/outlook express
server: Zimbra 7.1.2
DRBD+heartbeat for mirroing two servers mx1 and mx2
DKIM-milter
Users: 591

Situation:
1. mail size larger than 1.9M
2. Immediately after finishing downloading the large mail via POP3, the client (Zimbra Deskop or Outlook/Outlook Express) will disconnect from the server. I used tshark to collect some information and found the pattern, where 115.82.234.85 is a HSDPA random IP; and 192.1.1.6 is the server IP:

1515.717048 115.82.234.85 -> 192.1.1.6 TCP molly > pop3 [ACK] Seq=71 Ack=4196297 Win=11597 Len=0
1575.767331 192.1.1.6 -> 115.82.234.85 TCP pop3 > molly [FIN, ACK] Seq=4196297 Ack=71 Win=5840 Len=0
1575.767813 115.82.234.85 -> 192.1.1.6 TCP molly > pop3 [ACK] Seq=71 Ack=4196298 Win=65534 Len=0
1587.504447 115.82.234.85 -> 192.1.1.6 TCP [TCP Dup ACK 462502#1] molly > pop3 [ACK] Seq=71 Ack=4196298 Win=65534 Len=0
1645.151647 115.82.234.85 -> 192.1.1.6 POP C: QUIT
1645.151724 192.1.1.6 -> 115.82.234.85 TCP pop3 > molly [RST] Seq=4196298 Win=0 Len=0
1645.152163 115.82.234.85 -> 192.1.1.6 TCP molly > pop3 [RST] Seq=77 Win=8192 Len=0

After the above transmission, the client will lost connection from the server. For Zimbra Desktop, the client IP doesn't change. However, for outlook/outlook express, the Internet connection via HSDAP 3.5G will get disconnected. So weird.

It got even worse before I increase POP3 session pool from 300 to 600. The FIN ACK will not arrive on the server. That is, the email client will keep downloading the mail.

Mobile users cannot retrieve email via pop3 normally. Can any expert help me on this one?