I had a problem sending mail using xpress mail on the ATT Tilt. My xpress mail account is linked to our exchange server and was receiving mail with no problems, however when I sent or replied to mail, it remained in my Outlook outbox until I arrived in the office to hit Send/Recieve, even though it was set to send every five minutes. It took me many hours of searching forums and Live chats with ATT support to no avail. Finally I looked at my outlook settings and checked "send immediately when connected" and now all is well.
I just wanted to offer this simple solution in case anyone is experienceing a similar problem.