Email Services - Frequently Asked Questions:
Search the FAQ:
Why do I see a certificate error when using Outlook 2007?
When using Outlook 2007 users may see the following error:
This error is due to how Outlook 2007 processes SSL certificates. The recommended workaround is to
configure Outlook 2007 to connect to the host "mailbox.zimbra.upenn.edu".
Information Systems and Computing, University of Pennsylvania