It's true that California and federal law take the crime of mail theft very seriously.
Under Penal Code 530.5(e) PC, theft of US mail is a misdemeanor carrying a maximum county jail sentence of one (1) year. And you can be guilty of this crime not just for stealing someone else's mail--but for removing the contents of someone else's mail, or receiving or possessing stolen mail, as well.
But California mail theft law specifies that you are only guilty if you act knowingly. If you accidentally take or open someone else's mail--not an unlikely scenario given a mistake by the post office--you are not guilty of this crime.
And even if you did knowingly steal or open someone else's mail, you may be able to convince the prosecutor to drop or reduce the charges if you acted alone and did not commit or attempt any additional crimes.
Mail theft is taken seriously largely because it is a first step toward the more serious crimes of identity theft or credit card fraud. These days law enforcement is concerned about organized crime groups conducting large-scale mail theft operations in order to gather personal information for identity theft rings.
So if you committed mail theft randomly, or were motivated by personal concerns, you may be able to avoid the harshest penalties for this offense.