Mailbox Rule

The term “mailbox rule” is most commonly found in areas of contract law. The mailbox rule helps to specify just when an offer is deemed to have been accepted for the purpose of the contract, when the acceptance of terms is sent by mail. According to this rule, acceptance of the deal is effective at the time the acceptance letter, or other document, is placed – with proper postage and the correct address – into the mailbox. Service may also been deemed effective if the offer requires the letter to be accepted by personal delivery, rather than by mail, on or before the deadline. To explore this concept, consider the following mailbox rule definition.

Definition of Mailbox Rule


  1. A rule of law that holds that an offer of contract is effective immediately once acceptance is communicated, whether by mail or other means.


1818    Established by case law

What is the Mailbox Rule

The term “mailbox rule” is typically found in contract law, and it concerns the acceptance of a contract that is received by mail, fax, or email. It also applies to acceptance of an offer of settlement, and offers of employment, among other situations in which an offer is made, and accepted or rejected. Service, or communication of acceptance, is ruled to be effective when the letter of acceptance is placed, with proper postage, and correctly addressed, into the mailbox. Acceptance of the offer must be communicated in a timely fashion, and before the expiration date of the offer.

Acceptance may also be considered effective at the time the acceptance letter is personally delivered, if that is an accepted method, according to the parties. The mailbox rule is an exception to the general idea that a contract takes shape the moment the offeree agrees to the contract in a face-to-face meeting.

The following is an example of the mailbox rule insofar as how it would be expected to play out from the day the offer is mailed until it can be officially declared accepted in a court of law, should the need arise:

On June 6, Deborah, the offeror, sends a letter to Jeff, the offeree, offering to work as a freelance graphic designer for a project Jeff needs help with, for a certain amount of money. In her letter, Deborah states that Jeff has one week to accept her offer, or it may be changed or revoked. Jeff receives the letter on June 7 and, the following day, June 8, Jeff sends his acceptance of Deborah’s offer to her by mail. On June 9, Deborah receives his acceptance letter.

As per the mailbox rule, Jeff’s acceptance is considered effective as of June 8, due to Jeff’s reasonable and proper service of his acceptance of Deborah’s offer. This gives the two a basis for creating the actual contract. Jeff’s response was timely, since he was given one week to respond, and responded within only a couple of days. Even if Deborah had received the response after the one-week deadline had passed – after all, he did send it via “snail mail” – the offer would still be considered valid, because of the date on which Jeff mailed it out.

If, however, Jeff mailed his acceptance letter back late, it would not qualify as his acceptance of Deborah’s offer. Instead, Jeff’s response might be considered a counteroffer, which could be upheld as such in a court of law. Deborah would then be permitted to either accept or reject the counteroffer.

Applying the Mailbox Rule to Email Communications

When applying the mailbox rule to email communications, there is no difference between sending an acceptance letter via “snail mail” and sending it via email. The end result is the same: the acceptance letter must be marked and served timely and correctly. The key here is documentation of the date and time the acceptance was sent – whether by a postmark, or by a sent receipt from a fax or email.

Some countries have different rules when applying the mailbox rule to email communications. The best way for potential parties to a contract to learn about applying the mailbox rule to email communications is to simply look into whether it is permissible within the state where the deal is being made.

Related Legal Terms and Issues

  • Offeree – The person to whom an offer of a contract is extended, who can then accept or reject that offer.
  • Offeror – A person who makes an offer to another person, thereby establishing a contract.
  • Plaintiff A person who brings a legal action against another person or entity, such as in a civil lawsuit, or criminal proceedings.
  • Trial A formal presentation of evidence before a judge and jury for the purpose of determining guilt or innocence in a criminal case, or to make a determination in a civil matter.