A rule of contract law that makes an exception to the general rule that an acceptance is only created when communicated directly to the offeror.
An acceptance is binding and the contract is said to be perfected when the acceptor places this acceptance in the mail box for return mail even if, in fact, it never reaches the offeror.
Many jurisdictions refer to this as the mailbox rule but even in Canada, there is disagreement: Waddams, in The Law of Contracts refers to it as the "mailbox rule; but Fridman, in The Law of Contract, to the "postal acceptance rule".
An 1892 British case (Henthorn v Fraser 2 Chancery Division 27) summarized it as follows:
"Where the circumstances are such that it must have been within the contemplation of the parties that, according to the ordinary usages of mankind, the post might be used as a means of communicating the acceptance of an offer, the acceptance is complete as soon as it is posted."
For more on this case and others, and on the postal rule, see "Offer and Acceptance" in the Contract Law section.