If you bought the BlackBerry used, there may be an old IT Policy on it from the previous user. The most common service an IT Policy denies is the use of email accounts other than the corporate (BES) account.
You can find out if there is an IT Policy on your 8900 by going to Options > Status and typing "buyr" (without quotes). Three new lines will appear at the bottom. Data Usage, Voice Usage, and IT Policy. Under IT Policy it should say "disabled" if you don't have one. Anything else means one is in place and restricting the functions of your BB. Easily resolved: Remove IT Policy - BlackBerryFAQ