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.