First of all let me say that I am not yet 100% sure if this is a Zimbra problem. I am posting this in case it is a known issue and that someone can save me a lot of time in problem determination.

Setup is as follows:
ZCS 4.0.2 installed in Fedora Core 4 running in VMware Server using bridged networking (am going to change to a dedicated NIC is case this helps);

Zimbra accessed from both local network and via port forwarding firewall from the internet (IPCOP firewall)

Multiple clients attaching using Microsoft Outlook as a POP3 client, Outlook users are sending mail using SMTP with authentication from both internal network and from the internet.

All works fine most of the time; however:

I have one set of 10 external users that are all using Microsoft outlook in a single office. The office has a NATed connection to the internet (IPCOP). All clients are set to check POP3 mail every 5 minutes.

Several times a day on various user machines Outlook reports time outs when trying to connect with POP3 or with SMTP. If the users simply waits for about 30 minutes to an hour the POP3 / SMTP connections will eventually be successfull and their incoming mail will arrive and the outgoing mail will be delivered.

This problem has only started happening since the installation of 4.0.2. I never had this problem with 4.0.0.

I can reliably make the problem occur on a machine by clicking send / receive in Microsoft outlook – all is fine. Keep pressing Send / Receive several times in quick succession so that several request go in one after the other and after about the second or third request Outlook will report time outs. Give it 10 minutes or so and the client will successfully send / receive again.

I suspect that it might be caused by lots of POP3 / SMTP requests coming into Zimbra from the same IP address for different clients is a short period of time.

Any ideas?