this may help.
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# How do I add other email (POP) accounts to my BlackBerry?
This example uses GMAIL:
* Go to the BlackBerry login page.
* Click on Create New account.
* Type in your device PIN and ESN. You can find both of these by going into Options-->Status on the Handheld device.
* Follow the rest of the setup choices to finish setting up your BlackBerry Web Account. This will create an email account and address in the format of dcassese@vzw.blackberry.net (where dcassese is the username you used when setting up the account).
* Once you have your account, look for Add other email accounts. Then simply add your email address and login information.
NOTE: You may also want to read Configuring your email client: BlackBerry Internet Service at the GMAIL Help Center.
SourceDoing more searching, and it looks like it is specific to your ISP also.
To start aggregating email accounts (like Yahoo! Mail) onto your BlackBerry, you must first run a setup from the BlackBerry Internet Service client. You can access the Service client from your BlackBerry or from your desktop computer.
To access the Service client from your PC, you need the URL that is specific to your network service. If your service provider isn't listed, contact it directly for the URL that is specific to your service.
After you enter your service provider's Web address in your Web browser, the Service client asks you to log in.
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