The penalty for stealing someone's mail is a serious thing.
Theft of someone's USPS mail is a federal crime and if convicted you can face up to 5 years in prison or even 20 years in prison.
If you break into someone's mailbox then it becomes a more serious issue and you'll certainly be arrested and be booked into jail.
Then when sentenced you'll likely get at least 10 to 15 years in federal prison.
It's never a good idea to break into someone's mailbox or steal their mail because you'll be headed to prison which is not a nice place.