Rihanna has reportedly stepped up her security after receiving sexually explicit mail from one of her legion of male fans. Identifying himself only as “Buster,” the mystery fan reportedly sent letters and drawings depicting himself performing sexually violent acts on the singer. Rihanna is the latest in a long line of celebs to find themselves the object of unwanted attention from overzealous fans. Both Ashanti and her mother were recently stalked by a male fan who sent suggestive and threatening text messages to them and their family members. Ashanti’s stalker was eventually caught, tried and sentenced to two years in prison. No word whether Rihanna has actually taken the steps to get the police involved.