Military Law
I sent an e-mail to a Capt X explaining what his duties were and asked his commander to replace him with a NCO. Is this insubordination to an officer?
Re: Military Law
I think it could likely be considered to be insubordination, yes.
If you expect to have any kind of career inside or outside the military, I would suggest you use the following as a guide: if writing something makes you feel good and is likely to make someone else angry or upset, file it, and never send it.
I have never been sorry I did not send an email. I have frequently been sorry when I did send an email while angry or annoyed.
You should apologize profusely and explain that you were not thinking with military bearing. It would be a good idea never to repeat that behavior.