Rihanna is reportedly stepping up her personal security after receiving X-rated letters from a crazed fan. The singer is said to be shocked by the graphic fan mail and her rumored boyfriend, baseball player Matt Kemp, is believed to have stepped in to make sure the obsessed letter writer, who calls himself ‘Buster’, can’t get to her. A source told the Daily Star: “Matt has asked Rihanna’s team to be extra cautious since she was sent two very steamy letters. Her friends are also concerned about the situation. They spent a long time talking about it when they dined at the Spanish Kitchen in West Hollywood last week. They are really worried things with this guy are going to get out of hand.”