California government guide에 보면 "you should send Certified mail and keep return the receipt. 라고 적혀있는데 이메일에관한내용은 없습니다. 이메일은 당사자가 안열어볼수있으므로 법적으로 유효하지않을것같은데 다음을 참조해보시고 정확한 답을 주시면 감사하겠습니다.
In order to cancel, the buyer must sign and date the cancellation notice, which must state that the buyer is canceling the contract. The buyer should send the notice to the seller by certified mail, return receipt requested, at the address that the seller has given in the sale documents. The buyer should keep a copy of the notice for his or her own records, and to be able to prove that the notice was given and what was said.