Is it legal to put a card in someone's mailbox?

In the United States, placing a card in someone's mailbox is generally illegal unless you are the postal service or have the recipient's permission. Mailboxes are considered federal property, and only authorized personnel are allowed to place items inside them. However, you can legally leave a card on a person's doorstep or in their mailbox if it's placed in such a way that it does not interfere with the mail delivery. Always check local regulations for specific rules in your area.