The postal rule (of acceptance) is the fifth and last of the relevant sub-rules affecting communication of acceptance. It can be summarized as, “If, and only if, the postal service is an acceptable method of communication between offeror and offeree, a letter of acceptance takes effect immediately upon posting, rather than upon receipt, and acceptance is therefore taken to have been communicated (to the offeror) at the time of posting the letter.” The rule itself was established in the 19th Century when the only communication between mutable parties was to deal wit each other via mail. There was none of the types of communication that we have today. Thus contracts were created face-to-face or by correspondence.

The rationale for the creation of the postal law came form the case Adams v Lindsell which involved the sale of wool between two parties, the defendant wrote to the plaintiff about the selling of some fleeces and required an answer via the post. The response letter was misdirected and not received till three days later and the plaintiff posted there letter on the same day but was not received until four days latter. On the day before it arrived the defendants not having received a reply by the date they expected sold all of the wool. The defendant agued that there could not be a contract until the letter was received. The judgment of this was that if contracts were not created until the acceptance letter was received then it would be impossible for any contracts to be created through the post as the defendants were not bound by their offer until the answer was received, then the plaintiffs would not be bound until they had received word that the defendants had received their acceptance, and this could go on indefinitely. Instead it needs to be considered that the offer is being made the entire time the offer