Placing non-US Postal Service Mail into a mail box is a VERY minor violation of USPS postal regulations. I seriously doubt that you could get ANYONE interested in prosecuting that offense. Besides,...
Not unless it was, perhaps, mis-delivered to your box instead, and you are doing your neighbor a favor by doing so. US Postal Regulations prohibit the placing of anything into a mailbox EXCEPT those...
Yes, that is generally how the postal service works. There are conditions to this, however. The envelope will need to be addressed with the appropriate, legible receiving address, including the zip...