Entourage 2004, Comcast mail and Root Certificate Error

I am running Entourage 2004 on an older-model Mac operating on Mac OS X (10.6.8). When I launch Entourage, I get the following error message:

Unable to establish a secure connection to mail.comcast.net because the correct root certificate is not installed. If you continue, the information you view and send will not be secure.

I have reinstalled/updated the root certificate, which shows a 2020 expiration date. Any ideas?