Nolo's Plain-English Law Dictionary

Mailbox Rule

In contract law, the acceptance of a contract is effective when a properly prepaid and addressed letter of acceptance is posted, as long as it is sent within the time in which the offer must be accepted (and unless the offer requires acceptance by personal delivery on or before the specified date). The mailbox rule is an exception to the general principle is that a contract is formed when acceptance is actually communicated to the offeror.
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