Emailing other users
Functionality that allows for emailing other users is, sadly, ideal for Internet stalking/harassment, a far from uncommon occurrence these days. Indeed sites aimed predominantly at women could suffer more than most, and not all publicity is good publicity.
More likely still is this functionality being used to harass another user without the offensive content being removed by the moderators. Indeed one of our existing clients was recently contacted by a user threatening to go to the police over abuse suffered through their site. It transpired that once the offender had tired of the moderators removing their abusive posts, they had started abusing the victim more viciously via email. This made it particularly difficult for the client, as they had no evidence of what had actually happened.
Another question to ask is whether you actually want users conversing privately? It may just be better to have their general chitchat on your site to improve your post count and so aid the sale of advertising.
If you feel that users should be allowed to converse privately, then a compromise might be to allow users to give out their own email address on the site, stating in your House Rules that they do so at their own risk.
Chat Moderators currently provide human moderation services to brand owners across Europe and North America.