By default, clicking an e-mail link from within NetPositive will launch a new BeMail message. If you’re using Mail-It, Adam, or another BeOS e-mail client, this may not be the behavior you want. NetPositive, however, does not include an option in its preferences to change the default mail client.
There’s a good reason for that — what you actually want to do is to change the default mail association system-wide, rather than for NetPositive alone. You could do this via the FileTypes preferences panel, but you’ll probably find it easier to right-click on any BeMail message, choose Open With… from the context menu, and release there rather than choosing another app to open that particular message with.
In the yellow-backed dialog that ensues, select the mail client you want and click „Open and make preferred.“ Your system-wide mail client association will be changed permanently (or until you change it back again).

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Here’s a slightly different approach to the same basic situation. These instructions provided by BeatWare:

Making Mail-It the Default Mail Program

You can make Mail-It the default email package (i.e. the one that is started automatically from other programs such as NetPositive) by following these simple instructions:
1. Launch Mail-It. This lets the BeOS know that the application exists.
2. Close Mail-It.
3. Open the FileTypes application, which is available from the
Preferences folder (you can also find the Preferences folder in the
Tracker menu).
4. Select and expand the „text“ field from the list of file types located
just below the File menu.
5. Select „E-mail“ from the list of items listed under „text.“
6. Under „Preferred Application:“ select „Mail-It“ in the drop-down list.
7. Close the FileTypes window.