TMobile has a hybrid push mail that was available under WM5. Basically what happens is you give TMobile.com you email address, POP & SMTP settins plus password. They then monitor you inbox. When an email is recieved thier server generates a text message that triggers you phone to check your email. The text message is invisible to the user and is automatically deleted. The advantage is that you do not have to set you phone to check email every 5min, 15 mins etc. You phone only checks when triggered to do so.
Here is probably a better discription;