When under contract, post-mutual acceptance, the contract only ends upon fulfillment of the contract or upon termination. Prior to mutual acceptance, when the offer is still an offer rather than a contract, there is a different process.
Offers are presentations of a party’s intent to be bound to terms of a deal. Contracts are only binding when the parties intend to be bound to the same terms. If you want to back out of a deal or if you change your mind on the terms that are offered, the other party is not expected to read your mind, so there is a notice requirement to withdraw an offer. There is a unique rule that developed in the early 1800s called the “Mailbox Rule,” directly as a result of the inability to read minds in real time. In an old English case called Adams v. Lindsell, EWHC KB J59 (1818), one merchant, Lindsell, sent a letter to another, Adams, offering “eight hundred tods of wether fleeces, of a good fair quality of our country wool, at 35 shillings, 6 pence per tod.” The letter was sent to the wrong place and wasn’t received until several days later, and was immediately accepted on September 5 by Adams. Postage in those days took some time, and the acceptance letter was not received until September 9. Unfortunately, Lindsell thought the deal was a nonstarter because of the week of silence and sold the wool to another person on September 8th. The court found that the act of sending the acceptance formed the moment the acceptance was sent by post and left Adams’ hand [the moment the acceptance was posted] and therefore, the wool was officially sold to Adams on September 5.
The rule adapted slightly over time to reflect that rejection becomes effective upon receipt by the offeror [similarly, a withdrawal would be valid upon receipt rather than upon posting]. In Stevenson, Jaques, & Co. v. McLean, 5 Q.B.D. 346 (1880), McLean, “being possessed of warrants for iron” [evidence of ownership of the goods that are held in a warehouse], offered to sell to Stevenson & Co. for 40 shillings per ton. At 1:00 the next afternoon, Stevenson found a purchaser and agreed to the price offer, sending a telegram to McLean at 1:34 accepting his offer. However, McLean found a different buyer and telegraphed the withdrawal of the offer at 1:24 that same day, 10 minutes before Stevenson’s “acceptance” telegram. When McLean refused to honor the contract with Stevenson, the lawsuit followed. The court ruled that a person making an offer is considered to be “repeating the offer every instant of time” unless revoked [or until the expiration of the offer]. It was an established principle of law at this stage that uncommunicated revocations were not revocations at all, hence the judge inferred from the combination of the concepts that the offer is valid and able to be accepted until the revocation is fully communicated to or received by the other party.
In the digital era, this is a lesser issue because there are many fewer moments where there is a lag between the sending of a revocation and the receipt of the revocation. Nonetheless, it is important to know that your offer is valid until the other party learns that you withdraw it. To some extent, this is the value of the timestamp on a signature. The law does not strictly obligate parties to timestamp signatures, but if you can prove that your client’s signature and “we accept” document packet email were sent before you received the withdrawal email, the mailbox rule is at play, and the sending of the acceptance overpowers the rejection.