I've come to the conclusion, do not neg/tease while doing the emailing. I find it almost always goes badly. Now don't get me wrong, neg/teasing in person to build rapport is good. But online it always seems to be taken the wrong way. Here are some reasons why:
a) Low self-esteem.
Many of the women, have super low self-esteem for themselves. Just think of how many women lie about their profiles to some degree. Whether it's doctored camera shots, calling their body style "avg" etc. You know you are already dealing with self esteem issues.
b) lack of body language
No way to communicate that you are only kidding/joking around. You can use smilee faces like but that sometimes gets regarded as showing lack of confidence.
So, I conclude that despite how well regarded negs are to the PUA community, it is way too dangerous a game with online dating.