No, actually. It's not only illegal, it's also very inconsiderate.
No, it is not legal to open someone else's mail that was sent to your address.
I would say no, I'm pretty sure that you have to have their permission.
Yes, section 23 of the Postal Services Avt 1998.
No, keeping someone else's mail is an offense with the US Postal Service.
No, it is illegal to throw away someone else's mail.
Yes, It is a Federal NO NO to open someone elses mail. and to impersinate someone to a Bank its also Federal NO NO. You just dont do stupid things like that.
Not if they have requested you do that. It would be similar to sending mail In Care Of someone else. You cannot hold the mail without permission and must return it to the mail carrier.
No.
Yes, e-mail can be considered a form of business correspondence when the e-mail is written to someone in a business or when representing a business.
No, it is illegal to open someone else's mail, even if it is delivered to your house.
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 tampering with someone elses mail or mailbox is a federal offense.