Okay. Does this happen with other applications, or only for the e-mail applications when they are displaying the e-mail address above the bottom row of icons? If your e-mail address is long, then this would be normal (although not desirable) behavior.
You can change what is displayed however. For example, instead of displaying "firstname.lastname@example.org," you could have "Personal E-Mail" displayed instead. To do this, login to your BIS management portal. This is the BIS portal for MTN: BlackBerry Internet Service
Login here, and you will see a list of e-mail addresses registered with your device. Next to the e-mail address you want to edit, click on the "Edit" icon on the right side of the screen. From there, change the "Email Account Name" field to be whatever you want displayed above the bottom row of icons. Save your changes, and logout of the portal. The changes should be pushed to your device almost instantly.
Let me know if I haven't explained this clearly.