MSNBC loudmouth Keith Olbermann flipped out when he opened his home mail yesterday. The acerbic host of "Countdown with Keith Olbermann" was terrified when he opened a suspicious-looking letter with a California postmark and a batch of white powder poured out. A note inside warned Olbermann, who's a frequent critic of President Bush's policies, that it was payback for some of his on-air shtick. The caustic commentator panicked and frantically called 911 at about 12:30 a.m., sources told The Post's Philip Messing. An NYPD HazMat unit rushed to Olbermann's pad on Central Park South, but preliminary tests indicated the substance was harmless soap powder. However, that wasn't enough to satisfy Olbermann, who insisted on a checkup. He asked to be taken to St. Luke's Hospital, where doctors looked him over and sent him home. Whether they gave him a lollipop on the way out isn't known. Olbermann had no comment.