What is the Postal Rule under English contract law?

The Postal Rule is a principle of English contract law that provides that an acceptance of an offer sent by post is effective when it is posted, rather than when it is received. This rule applies to contracts formed by correspondence and assumes that the offeree will receive the acceptance in due course. The Postal Rule is considered an exception to the general rule that a contract is formed only when the acceptance is communicated to the offeror.