Users of Norton 360 report getting a popup warning when attempting to access Web Mail, showing:
"There is a problem with this website's security certificate."
"This organization's certificate has been revoked"
This is a bug in the Norton product causing a false warning, and it is not a problem with Web Mail. Norton has a fix for the problem available here. The fix seems to be simply run the Live Update in Norton, but see their support information at the link above for more details.