Is it legal to leave business cards in mailboxes?

No. In the United States, a mailbox is reserved for mail that carries postage and is delivered by an authorized carrier, so slipping unstamped cards or flyers inside is a federal violation. It can bring fines, and it tends to annoy the very people you are trying to reach. There are plenty of legal ways to get cards in front of the same audience.

• Hand cards out in person at events, counters, and meetings, which converts far better anyway.
• Ask permission to leave a small stack on business counters and community bulletin boards.
• Use a mailing service so printed pieces reach mailboxes the legal way, with postage and carrier delivery.
• Partner with nearby businesses for reciprocal displays instead of dropping cards uninvited.