Just a few things to consider here:
He needs to be very very careful about those little emails, even though the women joke and flirt back - you never know when one of them could call him sexual harrasment. Flirting in the workplace is common, kills time, helps people get through there day sometimes but an example like above with his remark then her telling him to behave - even though it was a friendly exchange - the last thing he would want to do is lose his job for it.
There are some men that are the biggest flirts, talk about sex , talk about every woman being so beautiful, make constant remarks about getting together and hooking up and if anyone ever took them up on it they would completely refuse, and it would probably even upset them. Some guys are very faithful, and play a game of being so flirty but wouldnt ever dream or even want to take things beyond that.
There are some men that are reserved and respectful, never flirt etc, but WILL cheat on their spouse given the opportunity.
The point.. you never know what a person is capable of until they actually do it, if his flirting hurts you, you should tell him that, not why its bad, or why he should stop - let him come to his own conclusions there after you explain how it makes you feel inside. I wouldn't stress yourself and worry with reading those little emails unless you have significant cause to think there is a problem in your marriage, he's losing interest in sex, he is keeping unusual hours, etc.. most womens guts will tell them if something is off.
If he has always been an outgoing flirt, he probably isn't going to change but he should tone it down a little for the first point I made up there. I think its a pretty common thing to creat sexual tension although almost anyone will advise against it, it never ends well.