Anyone know why?
Or have a fix?

The type of radio in all blackberry devices does not permit simultaneous connections. The same thing happens during a phone call.

Are you refering to the fact that you can't send from the BB when you are connected to PC as a modem? If so, previous answer is correct -- you can only use the radio for one function at a time.

Alternatively, if you are refering to the fact that you can't send e-mail from your PC while using the BB as a modem, that's another matter. Such a problem would probably be related to STMP server set up in your mail program versus the actual wireless server to which you are connected. More info is needed to troubleshoot -- e-mail program and set-up, wireless carrier, etc.