What a despicable letter.
The bullying accusations are bizarre considering that what's happening to Corbyn can easily be described as bullying. He doesn't use that description because he doesn't do personality politics, but the collusion of the "rebels" and the media to constantly batter him with lies in the press and heckle him from his own backbenches is definitely bullying. They're basically crying because they keep hitting him and he refuses to fall down.
I specially like this bit:
'Of course we would not have had a leadership election had Jeremy Corbyn done the honourable thing and resigned as leader of the party.'
which certainly illustrates that they wouldn't recognise 'honourable' if they fell over it.