Some points re the good Major:
There is no such thing as a private e-mail. Regardless of whether or not he was using a military bearer system, countless legal rulings have proven that once written and sent, it is fair game for the public domain. If it was intended to be private, it should have been delivered by the appropriate means as such. If you can't imagine the foreman of the jury reading out every e-mail you send, you shouldn't be sending them.
Annual performance appraisals, Mess nights, and conversations with the CO are all really, really good occasions to share your views with the leadership on how things are going, and hopefully get some feedback as to why. The way this e-mail has been exposed by the bottom-feeders is unfortunate, but wholly predictable, given the clumsiness and naiveté demonstrated here.
To have a pop at your colleagues is routine and expected, especially from those Army units where 'esprit de corps' is beaten into you from day one. But like all things, there is a limit to how far it should go. The further up the command chain you are, the less acceptable it is to level accusations of this nature at a service branch that it ultimately involved in providing protection to everybody in that Theatre.
He's not working for KBR organizing truck convoys and he's not there as a private citizen. He is a member of the British Armed Forces deployed on Operations, so he does not have the luxury of passing his opinion to all and sundry, intended or not, and he is certainly not the official MoD spokesman on the effectiveness of CAS Ops. When he is, he can comment.
There is some obvious support here for yet another service professional caught in a difficult position, but the situation is largely of his own making. You have never been at liberty to express the same individual freedoms as the rest of society when you are serving as an Officer in HM Forces, especially when you find yourself in some hell-hole, courtesy of political ineptitude and a lack of moral fibre by your leadership. If that's a problem for you, write the letter, and go.