From CNN.com

Bombs and chemicals sent through the mail are rare but terrifying. According to the US Postal Inspection Service, the postal service processes more than 170 billion pieces of mail every year and only a tiny fraction of those — 16 over the last few years — were investigated as mail bombs, says the US Postal Inspection Service. That, according to the agency, works out to be an average of less than 1 in 10 billion.

However, mail bombs can still be deadly, as in the package that killed a 17-year-old in Austin, Texas, earlier this year, or the Anthrax attacks in 2001.
Read more to learn what postal services and government agencies look for when determining whether a package is suspicious.