It appears there is a limit to the number of entries in the "Add to Spam" address list in the native email app. Additional addresses, once the limit is reached, crashes the app. Delete a couple and you can add a couple.
I guess for me its not a problem, as I always go into outlook and add the entries in the Junk folder used by the phone to the master junk list in Exchange, but I wonder if some smart person out there might know a way to increase to number allowed.
Guess I will go find the Google bug report system, as it might at least be more elegant about reporting the list to be full without crashing.