Is it illeagal to put a letter in someone's mailbox?

No, just to take one out.

HTH
Scott<>

It is NOT illegal.

In fact, I did that just the other day.

Some of my neighbour's mail got mixed in with mine, so I went to their house next door and dropped their mail in their mailbox.

I'm not 100% sure about this, but I think it is illegal to put anything in a mailbox that isn't official US Mail. Like for instance solicitors aren't supposed to put leaflets in mailboxes. If it's somebody else's letter that got mixed up with your mail that would be OK, but a letter you write to a person but don't put a stamp on it and mail it, just put it in their box yourself, I don't think so.

Well I did some research on this and found this article:

Whose Mailbox is it Anyway? [Mackinac Center for Public Policy]

Now I knew that the USPS does own the mailbox. But I never heardwhere it was illegal to deposit something other that US mail in one. So I looked up the code

United States Postal Service

And couldn't find anything prohibiting putting anything in a box as long as it doesn't damage the box or interfere with the delivering of regular mail.