Izrunas, I hope you got your problem solved. I am writing in case you haven't, and for the public benefit.
Zimbra 8 introduced a new feature to throttle POP3 and IMAP4 and it can cause problem to the users who check their email very frequently. The POP3 connection can be rejected by the server and the Outlook gives "connection interrupted by the server" message. /opt/zimbra/log/mailbox.log file shows similar entries as follow.
2012-10-18 10:53:37,828 WARN [Pop3Server-213] [ip=192.168.100.154;] pop - throttling POP3 connection for remote IP 192.168.100.154
In Zimbra 8 Open Source Release note, it writes as below.
I don't really believe the default values are the requests/second, it may be requests/minute. Some experts can verify and advise.
The current values can be checked with the following command.
I believe setting the value to 0 will disable, but I wanted to have basic protection against POP3 hacking attempts or DOS attack. Setting bigger limit (2,000 for example) solved the problem.
zmlocalconfig | grep -i throttle
Do not include space at the sides of '=' in the command.
zmlocalconfig -e imap_throttle_acct_limit = 500
zmlocalconfig -e imap_throttle_command_limit = 25
zmlocalconfig -e imap_throttle_ip_limit = 500
zmlocalconfig -e pop3_throttle_acct_limit = 2000
zmlocalconfig -e pop3_throttle_ip_limit = 2000