Contracts Made Clear: The Mailbox Rule and When the UCC Applies Instead of Common Law

Two early Contracts topics cause more confusion than they should: deciding which body of law governs a contract, and figuring out when an acceptance becomes effective. Both are rule-heavy and both show up on exams.

Step one on every Contracts question: which law applies?

The rule: Article 2 of the UCC governs transactions in goods. Goods are things that are movable at the time they're identified to the contract. Common law governs everything else, including services, real estate, and employment.

That's easy when someone buys a used car (UCC) or hires a painter (common law). It gets harder with mixed contracts.

Mixed goods-and-services contracts: Most courts apply the predominant purpose test. Ask what the contract is mainly about. A contract to install a new furnace, where the furnace costs most of the price, is predominantly for goods. A contract to repair a furnace, where a small part is replaced, is predominantly for services.

On an exam, state the rule, identify the facts that point each way (price allocation, how the contract describes itself, what the buyer really wanted), and reach a conclusion. That paragraph is often worth more points than students expect.

Why it matters: The rules differ. Under the UCC, a contract can be formed even if the acceptance adds terms (UCC 2-207), firm offers by merchants can be irrevocable without consideration (UCC 2-205), and modifications need no consideration if made in good faith (UCC 2-209). Under common law, the mirror image rule applies and modifications generally require consideration.

The mailbox rule

The rule: An acceptance sent by mail (or another reasonable method) is effective when it is dispatched, not when it arrives. This is the mailbox rule.

Other communications work differently:

• Revocation of an offer is effective when the offeree receives it.
• Rejection is effective when the offeror receives it.
• Acceptance of an option contract is effective on receipt.

The two classic mix-ups

Rejection sent first, then acceptance. The mailbox rule doesn't apply. Whichever arrives first controls. If the rejection arrives first, there's no contract.

Acceptance sent first, then rejection. The acceptance is effective on dispatch, so a contract exists, unless the offeror receives the rejection first and detrimentally relies on it, in which case the offeree may be estopped from enforcing the contract.

Offeror's control: The offeror can opt out of the mailbox rule by stating that acceptance is effective only on receipt. Watch for that language in a fact pattern.

A quick practice problem

On March 1, Seller mails an offer to Buyer. On March 5, Buyer mails an acceptance. On March 6, Seller mails a revocation, which Buyer receives on March 8. Seller receives the acceptance on March 9.

Is there a contract? Yes. The acceptance was effective on March 5 when dispatched. The revocation came too late because it had to be received before the acceptance was sent.

Where Lexora Prep fits

Timing rules like these are easy to confuse and easy to test, which makes them good candidates for daily recall practice. Lexora Prep is a Black Letter Law rule-training platform where you practice rules like the mailbox rule and predominant purpose test from memory, including an Ordering mode that checks whether you know which step comes first. Lexora tracks the rules you miss and brings them back until you get them right consistently.

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About Lexora Prep

Lexora Prep is a Black Letter Law rule-training and essay practice platform for law students and bar exam candidates. It gives you 700+ concise rules across 8 bar-tested subjects and asks you to recall them, not just reread them, through five modes: Typing, Fill Blank, Rule Elements, Ordering, and Flashcards. Lexora tracks the elements you miss, brings weak rules back for review, and builds each session around your weak areas and available study time. When you're ready to apply the law, Essay Practice gives you 120+ original essays with detailed feedback on issue spotting, rule accuracy, application, conclusions, and organization.

For law students, that means rule statements that are ready for finals, and a head start on the bar exam you'll take after graduation.

Start practicing free → The Free plan includes a rule-practice preview and 3 Essay Practice questions.

Lexora Prep is a supplemental study tool, not a full bar review course, and it does not guarantee exam results. Legal rules can vary by jurisdiction; verify important rules with your course materials.