Under the mailbox rule, an acceptance sent by mail becomes legally binding the moment it leaves the sender’s hands and enters the postal system, not when the offeror opens the envelope days later. The Restatement (Second) of Contracts § 63 states the principle directly: an acceptance is effective “as soon as put out of the offeree’s possession, without regard to whether it ever reaches the offeror.”1Legal Information Institute. Mailbox Rule A contract exists at the point of dispatch. If the letter is later lost, delayed, or destroyed, the agreement still holds.
Why the Rule Favors the Offeree
The logic protects reliance. Once you’ve mailed an acceptance, you may start arranging financing, hiring contractors, or turning down competing offers. The rule locks in the deal at dispatch so that reliance has a foundation. It also strips the offeror of the ability to pull the offer back once your acceptance is in the mail. If you post your acceptance on Monday and the offeror mails a revocation on Tuesday, a binding contract already existed before the revocation was sent.
The asymmetry is deliberate. The rule applies only to acceptances. Offers, rejections, counteroffers, and revocations all follow a receipt rule: they take effect when the other party actually gets them. The moment of acceptance is the one moment the law singles out for extra protection, because that is when both parties’ expectations converge.
What You Have to Do for the Rule to Apply
The protection is conditional. Sloppy mailing forfeits it, and the acceptance will not count until it actually arrives. Three requirements have to be met:
- Correct address. The envelope needs the recipient’s complete and accurate address, including street number, city, state, and ZIP code.
- Proper postage. You have to prepay the full postage for the item’s weight and size. Insufficient postage can get the letter returned or delayed, and the rule will not save you.
- Deposit with USPS. The letter must enter the custody of the United States Postal Service, either through an official blue collection box or by handing it to a mail carrier. Leaving it in your office’s outgoing tray is not enough.
The triggering moment is when the letter leaves your control and passes into the federal postal system. Everything before that, sealing the envelope, writing the address, is preparation, not posting.
When the Mailbox Rule Does Not Apply
Several situations override the default. Getting caught by one of these is the difference between having a contract and having nothing.
Option Contracts
When you have paid for an option that keeps an offer open for a set period, the mailbox rule does not apply. Restatement § 63(b) states that “an acceptance under an option contract is not operative until received by the offeror.”1Legal Information Institute. Mailbox Rule The receipt rule governs because the person who granted the option needs certainty about whether it was exercised before the deadline passed.
Offers That Require Receipt
An offeror can override the rule by writing it into the offer. Language like “acceptance must be received by June 1” or “this offer is accepted only when I receive your written response” switches the default from dispatch to delivery. Mailing on May 30 will not help if the letter arrives June 3.
Misaddressed or Improperly Sent Mail
Wrong address, insufficient postage, or failure to deposit the letter with USPS all strip away the protection. In that case, the acceptance becomes effective only if and when it actually reaches the offeror. A transposed ZIP code is enough to lose the rule.
Revocations of Offers
Revocations take effect only when the offeree receives them. If an offeror mails a revocation on Monday but the offeree mails an acceptance on Tuesday before the revocation arrives Wednesday, the acceptance wins. A contract formed Tuesday at the moment of mailing, and the revocation was not yet effective.
Rejection Followed by Acceptance
When an offeree mails a rejection and then changes course and mails an acceptance, the rule stops protecting the acceptance. Whichever communication the offeror actually receives first controls. If the acceptance arrives first, a contract forms. If the rejection arrives first, the later acceptance becomes a new counteroffer that the original offeror can accept or ignore. The practical lesson: if you have rejected an offer and want to reverse course, do not rely on the mail. Call or email to be sure the acceptance arrives first.
Proving the Letter Was Delivered
A related doctrine, the presumption of delivery, handles a different problem: proving the letter actually arrived. Once you show that a letter was properly addressed, stamped, and deposited in the mail, courts presume it reached its destination within the usual delivery timeframe. Federal courts have held that proof a letter was placed in a U.S. post office mail receptacle creates a presumption that it reached its destination in the usual time and was actually received by the person addressed.
The presumption is rebuttable. The person claiming they never got the letter has to do more than deny receipt. Courts have repeatedly held that a bare denial is not enough. The recipient typically needs specific, credible evidence of non-delivery, such as documented postal failures in their area, evidence of mail theft, or testimony from someone with direct knowledge of mail handling at the address.2U.S. Department of Justice. Matter of F-B-G-M- and J-E-M-G-, 29 I&N Dec. 52 (BIA 2025)
The strength of the presumption depends on how you mailed it. Certified mail with a return receipt creates what courts call a “strong presumption” because there is a paper trail showing attempted delivery. Regular first-class mail creates a weaker presumption, since the only evidence is usually the sender’s testimony about mailing practices.2U.S. Department of Justice. Matter of F-B-G-M- and J-E-M-G-, 29 I&N Dec. 52 (BIA 2025)
Certified and Registered Mail
If you ever need to prove you mailed something and that it was delivered, certified mail is the standard. The USPS return receipt gives you a signed record of who accepted delivery and when. Federal regulations governing proof of service state that “when service is made by registered or certified mail, the return postal receipt will serve as proof of service.”3eCFR. 45 CFR 1149.16 – What Constitutes Proof of Service
For tax filings, registered mail carries even more weight. Under 26 U.S.C. § 7502, USPS registration constitutes “prima facie evidence” that the document was delivered, and the registration date is treated as the postmark date.4Office of the Law Revision Counsel. 26 USC 7502 – Timely Mailing Treated as Timely Filing and Paying Prima facie evidence means the court accepts delivery as proven unless the other side comes forward with contrary evidence.
Short of certified or registered mail, other evidence can still support delivery: mailing logs with dates and addresses, affidavits from the person who prepared or deposited the mail, postage meter records, and testimony about standard office mailing procedures. The more documentation you create at the time of mailing, the stronger your position if delivery is later disputed.
Timely Mailed, Timely Filed for Tax Returns
The mailbox rule has a statutory cousin in the tax code. Under 26 U.S.C. § 7502, a return or payment that arrives after the deadline is still considered timely if the postmark falls on or before the due date. The requirements mirror the common law version: the document has to be deposited in the U.S. mail, in a properly addressed envelope, with postage prepaid, and the postmark must fall within the filing period.4Office of the Law Revision Counsel. 26 USC 7502 – Timely Mailing Treated as Timely Filing and Paying Mail on April 15 and arrive April 22, you are safe. Postmark April 16, you are late.
Section 7502 does not apply to filings with courts other than the Tax Court, to payments in currency (cash has to be physically received), or to documents the IRS requires to be delivered by some method other than mailing.4Office of the Law Revision Counsel. 26 USC 7502 – Timely Mailing Treated as Timely Filing and Paying
The Private Delivery Service Trap
The timely-mailed rule extends beyond USPS, but only to specific services from specific carriers. Under IRS Notice 2016-30, which remains in effect until the IRS publishes a replacement, only certain shipping methods from FedEx, UPS, and DHL Express qualify. The list is narrower than most people expect. FedEx overnight and two-day services qualify; FedEx Ground does not. UPS Next Day Air and 2nd Day Air qualify; UPS Ground does not. If you use FedEx Ground for a return that arrives a day late, you cannot fall back on the timely-mailed rule, even though FedEx itself is an approved carrier. The carrier being designated is not enough; the specific shipping method also has to be designated.5Internal Revenue Service. IRS Notice 2016-30 – Designated Private Delivery Services
When you do use an approved method, the tracking confirmation serves as prima facie evidence of delivery, similar to USPS registered mail. The date the private carrier marks on the shipping label functions as the equivalent of a postmark. Check the IRS notice before choosing a shipping speed when the deadline matters.
Email and Electronic Acceptance
The traditional mailbox rule was built around physical mail and its inherent delays. Electronic acceptance moves fast enough that the dispatch-versus-receipt distinction rarely has practical bite. Federal law ensures electronic contracts are enforceable: under the E-SIGN Act, a contract “may not be denied legal effect, validity, or enforceability solely because an electronic signature or electronic record was used in its formation.”6Office of the Law Revision Counsel. 15 USC 7001 – General Rule of Validity An email acceptance is generally effective when it enters the recipient’s mail server, not when the recipient opens it, so the gap between sending and receiving is usually seconds. If timing of an electronic acceptance matters, the safest approach is to state in the offer exactly when acceptance becomes effective.
Protecting Yourself in Practice
The mailbox rule is a default that works for routine transactions, but relying on it without understanding its limits is where problems start. If you are accepting an offer by mail and the deadline is tight, use certified mail so you have a postmarked receipt showing when you mailed it. If you are filing a tax return close to the deadline, use USPS or an IRS-designated private delivery service, and keep proof of the postmark. If you are responding to an option contract, mailing is not enough; the offeror has to actually receive the acceptance before the option expires. For anything where the stakes justify the cost, certified mail with return receipt gives you the strongest legal position: proof of when you sent it and proof that it arrived.