Are you wanting to stop it from charging? or stop it from witholding messages while connected?
You can't stop it from charging.
If you still want to receive your email while it's connected, open redirector settings and on the first tab is the option to disable message redirection when in cradle.
The 7200's dont have cradles per se, but older ones did and the term "In Cradle" has just stayed to mean connected to the computer.
I'm curious about why you wouldn't want it to charge when plugged in as well. You'll increase the life of your battery if you let it top off and charge any chance it gets.