The posting rules stem from a case in 1818 Adams v Lindsell This case provided that acceptance is complete on posting. This is an exception to the normal rule of contract where acceptance must be communicated to the offeror for it to be effective. It is important to note that for a contract to be formed three basic criteria must be met; namely there must be an offer, or something amounting to an offer; there must be an acceptace of the offer, or something amounting to acceptance; and there must be Consensus ad Idem or a meeting of the minds.  
      The rationale for the posting rules used to be that the post office was the common agent of both parties involved (refer to law of agency). This meant that by handing the letter to the post office the person was effectively handing it to the principle, or the actual person. The modern rationale from the case of Brinkibon v Stahag Stahl is that of convenience. When communicating over distances using methods which create some delay, one party is always going to have to be disadvantaged. 
      Following the original principle, many more have been developed in thecorpus of law that is the posting rules. These are; acceptance is complete on posting even if the letter is delayed in the post; acceptance is complete on posting even if the letter is lost and never arrives; If a perticular mode of communication is prescribed then acceptance is not complete on posting under the general rule; A letter must be properly desptached for acceptance to be complete on posting;for a revocation of offer to be effective, it must be communicated prior to acceptance; and many more cases relating to telegram, telex, telephone and other modes of instantaneous or neear instantaneous modes of communication. The general rule regarding instantaneous or near instantaneous modes of communication is that acceptance is not complete until received. 
     Discussion of the doctrine of estoppel must also take place when considering the posting rules. 
     There are currently no precedents pertaining to the posting rules and courier, 'txt', or email. Any novel cases must be resolved in light of technological advances, societal considerations, and the principles pronounced in Brinkibon v Stahag Stahl. These principles are that no universal rule can cover all posting rules cases and that new cases must be resolved by reference to the intentionsof the parties, sound business practice, and in some cases a judgement of where the risks should lie. 
      This is written from a New Zealand perspective. It uses language and terms used in New Zealand. The posting rules are the same as the "mailing rules" or the "postal rules". 
      This corpus of law, as with all, is immensely complex and is far more detailed than what I have written. This will need to be properly referenced and verified but it is all correct. Also it will need to be expanded. 
   SDC