Common law has a default rule for when acceptance becomes effective—but most contracts are better off specifying their own.
Under the “mailbox rule,” acceptance happens the moment it’s sent, not when it’s received. Mail a letter accepting an offer, and the deal is done before the other party knows about it. This applies even if the letter gets lost and never arrives.[1]
Why does this strange rule exist?
In Adams v Lindsell, a wool merchant sent an offer by post, then sold to someone else before the acceptance arrived. The court faced a problem: if acceptance only counts when received, the offeror would need to confirm receipt, and the acceptor would need to confirm that—ad infinitum.[2]
So common law picked a side: acceptance on dispatch. The risk of postal delays falls on whoever chose mail as the medium.
The catch: This rule only applies to slow communication. Courts have ruled it doesn’t cover telephone, fax, or telex—those require actual receipt.[3] Email remains legally ambiguous.
The fix is simple: Specify in your offer that acceptance is effective only upon receipt. One sentence eliminates 200 years of case law uncertainty. Without it, you’re governed by a rule designed for horse-drawn mail coaches.
Footnotes
[1] Household Fire and Carriage Accident Insurance Co Ltd v Grant (1879) 4 Ex D 216.
[2] Adams v Lindsell (1818) 1 B & Ald 681. https://en.wikipedia.org/wiki/Adams_v_Lindsell
[3] Entores Ltd v Miles Far East Corporation [1955] 2 QB 327.