I found that the default http inspect policy causes web mail systems like Yahoo, Gmail, HotMail, etc... to be blocked. The problem was that the PIX just generated a general TCP Deny log entry instead of stating the inspect policy was triggered.
In order to get webmail systems working I had to customize the http policy to allow non-complaint RFC 2616 POST's.
This must be a common issue for PIX users, but I did not see anything posted about it. If you ran into this problem or have a better solution please post.