Contracts/Mailbox rule

The mailbox rule is a term of common law contracts which determines when a contract has been formed where the parties are communicating via the mail. The basic thrust of the rule is that an acceptance of an offer that is sent before a revocation of the offer is received. However, if a communication is sent rejecting the offer, and a later communication is sent accepting the contract, then the first one to be received by the offeror will prevail.

For example, suppose A makes an offer to B on January 1; A then decides to revoke the offer on January 2, and puts a letter in the mail to B revoking the offer; however, B puts a letter accepting the offer in the mail on January 3, and does not receive A's revokation letter until January 4. Because B sent his acceptance before receiving A's revocation, the mailbox rule dictates that B's acceptance is effective. A will therefore be bound to the contract, and can no longer revoke the offer.

Suppose, on the other hand, that A makes an offer to B on January 1, and B decides to reject the offer on January 2, and puts a letter in the mail to A rejecting the offer; however, the next day B changes his mind and sends A a fax accepting the offer. In this situation, whichever communication A receives first will govern.

Under the mailbox rule, defective performance is an acceptance, unless accompanied by an explanation.