In my opinion, you shouldn't read your members pms. A fellow webmaster from this site sent PM spam to members on my site but he was reported. I didn't have to look at pms because people told me. That's what the report link is for. If you have a lot of quality posters, real people, they'll report any problems they have with you to you. By volunteering yourself to solve their issues with the other poster, you set yourself up as being too controlling or strict or nosey and rumors might spread that you are doing creepy stuff etc. I've seen that happen to other webmasters who got too all up in people's business. When it gets serious such as people threatening to sue the site or serious harassing, stalking, predator, hacking, child pornography and spam type issues then it's your responsibility to gain full control of your site to find out what's going on and handle the situation before it spirals out of control and you no longer have a site.