Legal Question in Business Law in California

"Written notice" does not mean written on paper, and it does not mean delivered by the U.S. Postal Service. If your customer sent you a written notice via email, I'd say it qualifies.

I might see things differently if I knew what else the agreement said. But that seems unlikely.

I agree with Mr. Hoffman. E-mail is almost universally accepted as being a "writing" under the law, unless a specific contrary provision is in a contract or a statute that applies to the situation.

In addition to what the other attorneys say, I might suggest that, in the future, if you want mailed notice, you state in your agreement that notice of cancellation be by certified return receipt mail or overnight mail with the recipient's signature. In that way, there's no question that you received it and when, and vice versa.

Emails qualify as a writing as defined under the Evidence Code. It would be sufficient unless the contract specified a certain type of writing.