What is the punishment for withholding mail from someone who is not a minor but no longer lives at their former address?

The deliberate withholding and/or opening of US mail that is addressed to another party is a violation of federal law.
The penalty for tampering with US mail is a maximum of 5 years in a federal facility and/or a $250,000 fine.
The US Postal Service takes mail tampering very seriously and strongly urges anyone who has been harmed in such a matter to contact the US Postal Inspectors Service.