It is all about intent. If you opened in good faith...maybe thought it was yours, etc...if you did it out of curiosity, spite, or greed...yes it is a felony offense.
Opening someone else mail is a crime. However, parents can open their kids' mail under 18.
It is a Federal crime and up to 10 years in prison and a $250,000 fine to hide keep or open others peoples mail without permission if you are reported you will want to return the mail to the post office. This is true even if the person the mail is addressed to lives in the same household.
It is a Federal Offense. Even the person who delivery your mail and keeps any mail or ceases to deliver that mail and keeps it can lose their job, fine or put in prison. If an envelope is addressed to two people then either of those two people have the right to open that mail.
In England and Wales, it is a criminal offence to interfere with other people's post without legal permission, excuse or authority (for example, a parent to open a baby's birthday card etc.. or post...
Technically, yes, it is. "Tampering with the US Mail." If the mail is delivered to your address but the addressee does not live there - simply mark it as "undeliverable" or "unknown" and return it to...