OK, my apologies!!! I clearly did NOT understand that you were talking about Messaging (even though your had that word pretty predominately in your question.....)....my head was reading MAIL!!!! So, MY BAD!!!
The only thing I can see in the other posts that I have read with this issue is that not only does "Group Messaging" need to be turned off, but also MMS Messaging needs to be turned off.
There seems to be a lot of confusion out there about how all of this dovetails together, so I think your best bet at this point, would just be to try the turning off of the MMS Messaging to see if it remedies the situation without taking away any features,
Again, sorry for the long way around to this - was just off on another planet, apparently....
Cheers,
GB