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
Users: 591

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 is a HSDPA random IP; and is the server IP:

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