The enterprise activation icon is for setting up your handheld with a BES. It probably popped up because your bis service book(s) were the victim of your freeze up.
There's one of two options. The mostly likely is you have a desktop[cmime] service book and when you're trying to send email from that service books is being used. The bb network has no BES or desktop redirector to send it to so it sends the message back to your handheld essentially saying, "Hold on to this until I know what to do with it." Down side is it'll never know what to do with it. Up side is, it just so happens to mean that the email you sent never made it to it's intended recipient... Wait, I guess that's a downside too. This is easy to check, options -> advanced options -> service book. Check what [cmime] service books you have. Sounds like you're with T-Mobile so you should have a
[cmime] for all accounts you've set up. If you have desktop[cmime] that's bad mkay.