No, other people cannot open your mail unless you give them permission to do so. This is a federal offense that is punishable in court.
It isn't ok for anyone to open anyone else's mail. Apart from being a criminal offence, it is an invasion of privacy.
Yes
Yes. US Postal Regulations refer to this offense as "Intercepting Mail Belonging to Another."
Yes, it is illegal for a parent to open mail addressed to their teenage child without permission. Mail tampering is a federal offense under the United States Code.
If the patient gives permission, then no. If they're doing it without the patient's permission, then yes, it almost certainly is.
Yes, It is a federal crime if there is no permission.
In most places it would theoretically be illegal for her to do so without his permission. However, in practice, it's unlikely that she'd be prosecuted for doing so; she'd normally be considered to be acting as his agent (much as a secretary might open email addressed to her boss).
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.
No, not without your permission.
Open source
yes, but it would be a federal crime in the USA unless you had his or her permission.
They could but it is illegal without your permission.