We have had a problem in one of our Warsaw offices where their Entourage 2008 clients (connected to Exchange 2007 via OWA) were not able to send mail.
After much trial and error we found that the MSS packet size was set incorrectly. Allowing larger packets resolved the problem.
We set the MSS packet size to 1300 and the internal and external MTU size to 1500
Doing a tcpdump and searching for "MSS" found that the packet size was 1460. However it looks like the tcp packet length to the OWA server is 1400.
