The postal service also has detailed regulations on what and how to mail stuff. Explosives, ammunition, gasoline, and air bags are prohibited. Other items are restricted, and may have to be clearly marked or shipped in limited quantities. These include firearms, propane, alcohol, and prescription drugs.
What About Threatening or Suspicious Letters?
Threatening, disturbing letters sent through the mail can constitute criminal threats, terroristic threats, stalking, and similar crimes related to making threats. That's the kind of behavior lower-level criminal offenses are designed for, to stop someone before threats become attacks, and are illegal under state laws. Federal crimes cover some specific threats, such a threatening the President or other officials.
What About Harmless, Suspicious White Powder?
Turns out, there's a handy federal offense covering this sort of thing. 18 U.S.C. section 1038 makes it a crime to "engag[e] in any conduct with intent to convey false or misleading information [that] may reasonably be believed [to indicate] that" a biological agent is present. It also makes doing so a civil action, i.e. allows someone to sue.
Which is lawyer-talk for: no, you can't send threatening letters and add sugar, flour -- or corn starch -- to make it legal.