Key Takeaways

  • The mailbox rule, also known as the posting rule, states that acceptance of an offer is effective once it is sent, not when it is received.
  • It serves as an exception to the general contract rule requiring acceptance to be communicated directly to the offeror before it becomes binding.
  • This rule can apply to traditional mail and, in some cases, electronic communications like email, though application to digital channels is still debated.
  • The mailbox rule affects disputes over timing, such as when acceptance and revocation cross in transit.
  • Certain conditions limit its application, including when the offer specifies a particular method of acceptance or when instantaneous communication methods are used.
  • Courts in different jurisdictions may interpret or apply the rule differently, especially in modern contexts involving technology.

The posting rule, also known as a mailbox rule, is used to decide the time that an offer has been accepted, which is an important component of contract law.

Basics of the Posting Rules

One of the most controversial parts of contract law is issues related to postal rules. Because the communication involved is long-distance, it can be tricky to know when exactly a contract has been formed when using the mail.

Since many business parties are not able to meet in person, communication by mail is very common when negotiating a contract. Most of the questions involving remote communication and contract law center around when contracts are offered and accepted. Postal rules were written and adopted to help resolve issues related to offering and accepting contracts by mail.

Due to advancements in technology, there is also some debate of whether these postal rules apply to electronic communication, such as email.

The basis of contract law is offer and acceptance, determining when a contract exists between two parties. The posting rule is an exception to the normal rules for acceptance, which state that acceptance only occurs when it is directly communicated to the party that made the offer. Under the posting rule, a contract is considered binding the minute the acceptance is placed in the mailbox. This is true even if the person that made the offer never actually receives the acceptance.

The posting rule takes effect from the moment communication of acceptance is intended, including communication by mail or email. The mailbox rule is also used to determine when a communication can be considered delivered. In some cases, the deemed delivery date can be different from the date the letter was actually received. This rule also determines whether a letter of acceptance or one of revocation takes precedence.

Imagine that person A decides to make an offer to person B on April 1. The next day, person A changes their mind and sends a letter revoking their original offer. Person B sends their acceptance letter on April 3 and doesn't receive the revocation letter until April 5. The acceptance letter, because it was sent before the revocation letter was received, is the one that takes effect. Person A is not legally bound by the contract.

Now imagine a different scenario where person B initially rejects the offer by sending a letter and then decides that they want to accept instead. By faxing or emailing an acceptance before the letter arrives, the offer will be accepted since these communications will arrive first.

Performance also counts as acceptance under the posting rule. For instance, if one party orders 1000 units of a product, and then the second party mails the order, this is considered acceptance. Even if the performance is defective, it still counts as acceptance unless an explanation for the defect has been provided.

If person A puts in an order for 500 blue widgets and person B ships 500 red widgets, this is acceptance even though the color was incorrect. Should person B include a note explaining that they shipped red widgets because the no longer produce blue widgets, this is an accommodation instead of acceptance.  

Historical Development and Legal Basis

The mailbox rule originated in early 19th-century English law, most notably in Adams v. Lindsell (1818), where the court held that a contract was formed when the acceptance letter was posted, even though it arrived after the offeror had assumed there was no agreement. This decision created a binding precedent that shaped contract law in common-law jurisdictions, including the United States.

The rationale was to promote certainty and fairness in contract formation when communication delays were inevitable. By fixing the moment of contract formation to the time of mailing, parties could rely on a predictable rule rather than being subject to delays or misdeliveries beyond their control.

Courts have since refined the doctrine. For example:

  • Uniform Commercial Code (UCC) provisions incorporate the rule for certain commercial transactions.
  • Restatement (Second) of Contracts § 63 explicitly adopts the rule for non-option contracts, clarifying that acceptance is effective upon dispatch.
  • Exceptions apply when the offer expressly requires receipt of acceptance or when the method of acceptance used is unreasonable under the circumstances.

Emails and the Posting Rules

Thanks to new information technology, people are able to communicate across large distances more easily than ever before.

Both businesses and individuals have access to multiple communication methods, including the following:

  • Telephones
  • Texting
  • Instant messaging
  • Email

There are many different options among legal professionals about which rules of acceptance apply to electronic communications. For example, acceptances made on website depend on the space between the person making the offer and the person accepting, as well as the time the offer was made.

Offers made through email are treated very differently than contracts formed via a website. There is virtually no established law related to accepting or revoking a contract offer through email. When sending information by email, the moment the send button is pressed, the message is considered delivered. Although there is no law related to the issue, postal rules will generally apply to contracts offered by email, meaning as soon as the acceptance is sent, the contract is binding.

Jurisdictional Variations and Controversies

Different jurisdictions interpret the mailbox rule differently, particularly when applying it to electronic communications:

  • United States – Most states follow the Restatement’s approach, but court interpretations vary on whether the rule applies to emails and other near-instant communication.
  • United Kingdom – The rule still applies to letters but generally does not extend to instantaneous communications like telex, fax, or modern messaging.
  • International Contracts – Under the United Nations Convention on Contracts for the International Sale of Goods (CISG), acceptance is effective upon receipt, not dispatch, unless the parties agree otherwise.

Controversies often center on balancing the predictability of the traditional rule with the realities of rapid modern communication. Critics argue that in an age of instant messaging, the rationale for the rule is weaker, while proponents maintain it still provides valuable certainty in asynchronous communications.

Conditions and Limitations of the Mailbox Rule

While the mailbox rule often favors the party sending acceptance, it is not absolute. Its application depends on several factors:

  1. Authorized Method of Communication – If the offer specifies a required method of acceptance, the mailbox rule applies only if that method is used. For example, if an offer demands acceptance by courier, mailing a letter would not trigger the rule.
  2. Reasonable Dispatch – The acceptance must be sent within a reasonable time frame. A delayed or improperly addressed acceptance may not be valid.
  3. Revocation Exceptions – If an acceptance is sent after a revocation is received, the revocation will prevail.
  4. Option Contracts – The rule does not apply to option contracts; acceptance is effective only upon receipt.
  5. Instantaneous Communications – For phone calls, faxes, and certain electronic communications, courts generally hold that acceptance is effective upon receipt, not sending.

Practical Implications in Modern Contracting

In business transactions, the mailbox rule can influence strategic decision-making. For example:

  • Risk Allocation – The offeror bears the risk of delayed or lost acceptance in transit.
  • Contract Disputes – In litigation, proving the date and method of dispatch can be critical evidence.
  • E-Commerce and Digital Acceptance – While traditional rules were built for postal systems, many courts analogize emails to letters, treating them as effective upon sending if the parties have a history of email correspondence. However, this remains an evolving area with no universal standard.

Businesses can manage uncertainty by:

  • Stating in offers that acceptance is only valid upon receipt.
  • Using tracked or verifiable delivery methods.
  • Implementing automated confirmations for online transactions.

Frequently Asked Questions

  1. What is the main purpose of the mailbox rule?
    It establishes that acceptance is effective upon dispatch, providing certainty and fairness when delays in delivery occur.
  2. Does the mailbox rule apply to emails?
    Often yes, if emails are the agreed method of communication, but courts vary in their treatment of electronic communications.
  3. Can an offeror avoid the mailbox rule?
    Yes, by explicitly stating in the offer that acceptance is only valid upon receipt.
  4. Does the mailbox rule apply to option contracts?
    No, for option contracts, acceptance is only effective upon receipt by the offeror.
  5. What happens if acceptance and revocation cross in the mail?
    If the acceptance is dispatched before the revocation is received, the acceptance typically prevails, and a contract is formed.

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