Hi, I have made letters which are handwritten and made copies of them(hole punched on regular lined paper). As I am driving for dollars, I wanted to leave these handwritten notes in people's mailbox. My husband has advised me that this may be illegal. Do you guys do that? Where could I leave a note saying I'm interested in their house?Must I leave it by their door?

Using mailboxes for anything but mail is technically illegal, but I doubt anybody would do anything about it. However if you have a note in the box and Mr. or Ms. Postman goes to deliver the mail, they will either pull it out and trash it or they may just pile everything on top (depends on their work ethic).

yes it is illegal to put anything inside someone's mailbox.  You do not want to be caught opening someone else's box it is a federal crime.  You can tuck it behind the flag or if they have a second newspaper box you can use that.  I would strongly advise against opening someone's mailbox.  

Thank you Bryan.Yes you are right!

I would recommend doing as @Bryan Chuchta suggested.  Put them in somewhere near the door handle (punch a hole in the corner and tie a loop with string) so you KNOW they will see it when they go to open their door.