Many people try to mail things for free. There have been all sorts of attempts; cancelled stamps, less postage, no stamp (oops! it fell off I guess), postmarking an unstamped letter to its destination, and other scams attempted by poor and smart college students. I used to work in the receiving room for my university, and dealt with the Post Office a lot. I have seen many things come off that little white truck, in many different conditions.

Why these and other attempts will usually won't work:
  • The items are postmarked for where they came from. Usually places will turn a raised eyebrow to people sending things across state lines.
  • A lot of items are scanned by machine for postage. The Post Office, from what I have heard, does most of its scanning by hand. Therefore people don't actually look at the letter until something has gone through there. If the scanner misses something, then it won't show up.
  • Stamps aren't lick and stick anymore: Almost all stamps (on all domestic items, at least) are self-adhesive. This means they are considerably less likely to be licked incorrectly, and thus likely to fall off (although during the initial run of self-adhesive stamps, many fell off due to people licking them).
Why this has, and will continue to work:
  • The post office is run by people: Because people generally give people the benefit of the doubt, the mail will generally go through, unless they are cracking down on mail fraud, or near a stamp price change, when they are looking for the stamps in particular. Near a (my) college for instance, you will see them all get returned, because they are wise to students being generally fraudulent little bastards. We used to get many mails returned (phone bills and the like) due to no stamp. People usually forgot, but I'd bet more than few tried to defraud. In a small rural setting, a lot of postal people would let your letter to grandma slide if the stamp "fell off".

A few years back, Channel 3, a new station very much into investigative reports sent ten letters to random people from random addresses with one cent stamps. Nine out of ten made it with no problem, the other went with a "postage due" distinction crossed out on the back. Of course the Post Office was unavailable for comment, but it just goes to show that people can be human, and simply don't expect other people to be fraudulent.

The problem with taking advantage of the system makes them tighten it up for people who honestly make mistakes (as simple as forgetting a stamp, or as complex as licking a self-adhesive one). There are other ways to get around postage, such as have your workplace meter it, or what not. Then again, paying the 34 cents (or 50 at a machine) really isn't that bad, is it?

Mail fraud is a Federal crime, and carries a term of no more than five years, and no more than a $1,000 fine.

neil has informed me that the applicable postage laws for fraud are USC 1341, 1346, and possibly 1725, and that the above sentence is a tad harsh for this form of petty postal fraud.