This is your answer:
1) On the BIS Service, you can choose a name and reply address for your BlackBerry so all email you send out can say it is from your corporate email address...
2) Get the Exchange Admins to remove your email address from your mailbox. They then need to create a group and make your mailbox a member of it. When this is done, they need to create an external SMTP contact and use your custom blackberry email address for that (you can create it on the BIS screens... Will be something like
). Give the address to the admins of Exchange to then add as that external SMTP contact and then add that contact to the group which contains your Email Address....
With the above done, you will have the most logical solution for your issue.