When it comes to tools for slicing open that mail, it really doesn't get any deadlier than the Cas Hanwei 1918 Mark 1 Letter Opener.
For a bit of background, the 1918 Mark 1 was actually designed for close combat during World War I, but was only used during World War II. Now this vintage piece has been given a less violent purpose in the form of the Cas Hanwei 1918 Mark 1 Letter Opener. The blade is made from steel and is definitely more than sharp enough for its users. Below that is a hilt and a blackened bronze grip in the shape of a brass knuckle.
The Cas Hanwei 1918 Mark 1 Letter Opener will probably be the most badass mail slicer anyone will ever come across. Just make sure you use it specifically for letters.