Mailbox Rule Definition: A contract law exception that makes a mailed acceptance of an offer valid as of posting. An alternate term for the postal rule (for eg, used in the USA); when mailed, an acceptance of an offer to contract is valid from the moment it is deposited into a mailbox; on dispatch. This will yield to any prior stipulation as to other form of delivery set out in the offer or, if a second-thought rejection of the offer (or revocation of the acceptance), is later sent to the offeror but received before the acceptance, it nullifies the mailbox-ed acceptance even if that acceptance is later received by the offeror. Categories & Topics: Contract Law Dictionary Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!