no it is illegal t open someone else's mailbox
No you can not. The mailbox is for use of official mail that has gone through a post office only. I know of some instances that a mail carrier will not take mail from a box to be mailed if it does not have the name and address of the box owner on it. Meaning that you could not put your outgoing mail in someone elses box to be picked up.
It's actually against the law to open someone else's mail. You can put it back in your mailbox so it can be sent to the right address, or you can get a pen & write 'return to sender' on the envelope.
US postal regulations state nothing but US mail can be put into a mailbox
Yes. It is illegal for you to open anyone else's mailbox for ANY reason. You are not allowed to put anything into someone's mail box, either.
Yes, it is a violation of US Postal Service regulations (federal law). You may NOT put anything on or in a mailbox (even a so-called "private" mailbox on someone's home) other than US mail.
Put it back in the mailbox and send it.
What kind of mailbox? A residential mailbox? A rural delivery mailbox? A mailbox in a parcel store? An official USPS mailbox? All are offenses, the last one especially. The fact that it turned out to be empty is immaterial. You didn't know it was empty when you broke into it, your intent was to open it and gain access to whatever was inside.
Yes, as long as it isn't stolen mail. No, it is illegal, even to place a flyer on the flag. The mailbox is US Government property.
get an envelope put your mail the mailbox in buy a stamp, and hopefullly it gets where you need it to
Yes. If it came to you by accident and you know it's not your mail, you should put it back into the U.S. mail system. At least cross out the address if it is "your" address but some total stranger's name.
yes
If you let them, sure. If someone is doing this and you don't like it, ask them to move it and if they refuse talk to a lawyer. If you let someone put their mail box on your property and keep it there for some length of time you may be creating an "easement" granting them the perpetual right to keep their mailbox there.