ive been there :)
You just detach - dont rise and repeat what you have said.
You arent being unreasonable, keep the emails, including the one you have sent - if things end up in court, or mediation or somethng, then you have those to show its not you.
if he says ' fine - you dont care as a mother' or whatever - you just reply and say that thats great - thanks for letting you know, and you will no longer be providing the children with x, x and x when he is spending time with him.
Just dont bite. pretend its a business transaction with a 3 year old. whos in the middle of a tantrum.
You can be in control here - you just have to chose to be.
If he persists with the agression/ abuse/ being difficult, dont be afraid to tell him, via email, that this is not co parenting in your childrens best insterests, and that its abusive to you and harrassement, and that it is not acceptable, and should he keep it up you will be seeking legal advice.
You have to stand firm, show him you arent taking the shit, dont get drawn in and dont budge.
Ive been seperated for 4 years, i was back at the solicitors this summer and ex husband got all difficult again - a slow increase of arse behaviour, climaxing in a door step argument at my mothers where he told them if i ever disagreed with him he will always shout me down and that he will hate me till he dies. 6 hours arguing later - and a trip and aletter from the solicitors from me, i asked him to meet and come up with some plan to move forward as it was not acceptable and could not go on for the bext 12 or so years. Things are currently fab, im not naieve enough to think it will lastforever, but i needed to make a stand ( againn) to show him i wasnt going to be walked over and take it.
You can do it too..