I'm not sure if anyone can help with this but I figured it was worth a shot. I have a user that has a Zimbra e-mail account and he uses MAPI to connect it his Outlook 2010. Everything was working fine up until a couple of weeks ago. Now, when he is on our network, he can no longer send attachments through his Zimbra e-mail account. The system just hangs and then comes up with a network error.

If he disconnects from our network and connects to any other network (verizon, cable,etc) attachments can be sent just fine.

The problem only occurs on this one particular network.

Any suggestions on what may be causing the problem or troubleshooting tests we can try to solve this? We have disabled his anti-virus and done all the routine checks but he is still unable to send attachments when connected out to this one particular network. Very puzzling!