Fixed the issue: https://code.google.com/p/reviewboard/issues/detail?id=3438
The fix itself is actually depends on the implementation of

djblets/auth/views.py:register(). Currently this fix seems

to be the only way of checking if registration was successful.

But, I'm not sure if it is the best way to do it. I think

djblets/auth/views.py:register() should have a way of reporting

the status of registration to the caller.

Unit tests pass.
Did manual testing to make sure the emails only get sent after successful registration.
No automated testing done.