I am having a very odd issue and I believe it has something to do with the Zimbra server.

If I send a 3MB attachment to a receipient and that reciepient uses Outlook via IMAP or POP3 the attachment causes a timeout. With POP a few tries will eventually retrieve the attachment, but with IMAP it continues to fail.

I have two IMAP server running on the same VMWare server one is an Exchange 2003 FE server and the other is a Zimbra 3.1.4 on CentOS 4.3.

The Outlook 2003 client is connecting over the Internet (works fine via the LAN) when connecting to Zimbra it errors, I CC the same message to my Exchange server and Outlook has no trouble with it. Outlook is configured with two profiles one for Zimbra and one for Exchange. If I access the same Zimbra mailbox via HTTP the attachment downloads in ~5 seconds at a rate of ~750KB/s.

If I do a tcpdump on the Zimbra server everything looks normal except this.
15:59:09.844747 IP zimbra.dsm.net.imap > rrcs-67-78-21-74.se.biz.rr.com.2147: . 926489504:926490884(1380) ack 1082144614 win 5840
15:59:09.864039 IP rrcs-67-78-21-74.se.biz.rr.com.2147 > zimbra.dsm.net.imap: . ack 1380 win 65535 <nop,nop,sack sack 2 {1700630289:1700681349}{1700623389:1700628909} >
15:59:09.864091 IP zimbra.dsm.net > rrcs-67-78-21-74.se.biz.rr.com: icmp 68: host zimbra.dsm.net unreachable - admin prohibited
I am not sure what is causing the icmp 68 request, but I don't allow icmp in on my firewall.

Now for the weird part, if I connect to the Zimbra IMAP service on my MacBook Pro using the same Internet connection and Apple's Mail.app I am able to download the attachment without any problems.

The problem is of course that everyone uses Outlook and now they are forced to login to the web interface to download their attachments.

I haved searched through zimbra.log, but haven't found anything relavant.

Thanks in Advanced.