Is it illegal to put flyers in mailboxes even if they're for a nonprofit or community event?

Yes, it is illegal, and there is no exception for nonprofits, churches, schools, or community groups. Under federal law, a residential mailbox is reserved for mail that has carried postage through USPS. Placing any unstamped item inside is a violation no matter how good the cause, so use distribution methods that do not touch the mailbox.

• No nonprofit exception: intent does not matter, the rule applies to everyone.
• Mailbox is USPS territory: only postage-paid mail belongs inside.
• Legal alternatives: door hangers on the knob, community bulletin boards, and postage-paid mailings such as EDDM.