I've also been having problems with BMW Office, iPhone4S, iOS6, and receiving texts but not emails through iDrive.
I have found an unsatisfying workaround: After Bluetooth pairing and configuring and receiving a text to test it works in iDrive, go to Notification Center-->Mail and turn notifications off, then back on. Exit out, lock the phone, and send yourself a test email. For me, it goes through the iDrive. The bad part is, once you turn the car off and iDrive unpairs from the iPhone, the fix is gone--I have to go back to notification center and do the on/off thing again.
That seems like an iOS bug to me.
For those of you reliably getting emails to work without magic, I'd love to know about any differences in settings, etc. I thought maybe it was because my primary email account is an MS Exchange account at work, which forces encryption policies on the iPhone, but I deleted the account, used a different email address, and still had the same problem.
Any other thoughts?