Despite today's news, a mail bomb is still an extremely rare occurrence -- and that's why it's noteworthy that officials are dealing with several potential bombs today.
Here's how rare it is: According to the US Postal Inspection Service, the postal service processes more than 170 billion pieces of mail every year. Over the last few years, the number of mail bombs inspectors have investigated? An average of 16. That, according to the agency, works out to be an average of less than 1 in 10 billion.