Mail Fraud is prosecuted under 18 U.S.C. § 1341.  It is essentially the same crime as Wire Fraud, which is prosecuted under 18 U.S.C. § 1343.  The only real difference between the two crimes is the communications method.  The actual language of section 1341 is:

Whoever, having devised or intending to devise any scheme or artifice to defraud, or for obtaining money or property by means of false or fraudulent pretenses, representations, or promises, or to sell, dispose of, loan, exchange, alter, give away, distribute, supply, or furnish or procure for unlawful use any counterfeit or spurious coin, obligation, security, or other article, or anything represented to be or intimated or held out to be such counterfeit or spurious article, for the purpose of executing such scheme or artifice or attempting so to do, places in any post office or authorized depository for mail matter, any matter or thing whatever to be sent or delivered by the Postal Service, or deposits or causes to be deposited any matter or thing whatever to be sent or delivered by any private or commercial interstate carrier, or takes or receives therefrom, any such matter or thing, or knowingly causes to be delivered by mail or such carrier according to the direction thereon, or at the place at which it is directed to be delivered by the person to whom it is addressed, any such matter or thing, shall be fined under this title or imprisoned not more than 20 years, or both. If the violation affects a financial institution, such person shall be fined not more than $1,000,000 or imprisoned not more than 30 years, or both.

As you can see, the language is convoluted and it doesn't really tell an individual--who is presumed to know the law, after all--anything.  Depending on in which jurisdiction the case is brought, section 1341 is saying:

It is a crime punishable by up to twenty or thirty years of imprisonment (depending on circumstances) for a person to:

• Develop (or intend to develop) a plan to
  • defraud; or
  • get money by false or fraudulent methods; and
• Use the mails to carry out that plan.

The devil, of course, is in the details:  what's a plan to defraud? what are false and fraudulent methods? what does it mean to use the mails? what does it matter where the case is brought? what's the likely sentence for a conviction?  That's why it is especially important for an individual facing federal wire fraud charges to speak with a lawyer deeply familiar with federal criminal defense.