Dispatch rule is a principle of contract law that an acceptance becomes effective and binds the offeror once it has been properly mailed. However, if the offer provides that an acceptance is not effective until received then, the dispatch rule does not apply.
It also refers to the principle that when a pleading or other document is filed or served by mail, filing or service is deemed to have occurred on the date of mailing. However these rules vary from jurisdiction to jurisdiction.
Dispatch rule is also known as mailbox rule.