Which of the following is an exception to the mailbox rule?

• A. The offer specifies differently.
• B. The mailbox rule applies to all acceptances.
• C. The contract must be in writing.
• D. Acceptance by mail is never effective.

Answer: (A)

The mailbox rule makes an acceptance effective at the moment the offeree properly mails it, as long as the mailing is in an authorized mode. But the moment this rule does not apply is when the offer itself says otherwise. If the offer specifies that acceptance is only effective when received, or requires a particular method or condition for acceptance to be valid, those terms control over the general rule. That’s why the statement that the offer specifies differently is the exception: the contract terms override the default rule about when acceptance becomes effective.

The other statements don’t describe an exception to the mailbox rule. The rule isn’t universal to every acceptance without regard to how the offer invites acceptance. The requirement that a contract be in writing isn’t about when acceptance becomes effective. And acceptance by mail isn’t never effective; it is typically effective on dispatch unless the offer directs otherwise.