Is the default signature of BlackBerry's email "from my BlackBerry" or "This email was sent by BlackBerry"? Or what?
Enter "Message" (envelope icon), press the wheel to open the menu, select "Options", press the wheel to enter, then select "Mail Settings" and enter, set "Use Automatic Signature" to "Yes", and then enter the contents in the following text box, such as: Send via blackberry.