If you have updated the billing information in your account you will receive an e-mail informing you of this success.
On the other hand if you receive this e-mail and have not made any changes to your account we always recommend checking out your account using the methods described here:
This is usually a result of your card issuer pushing new card details to our payment processor. Since if you have a subscription with us, our payment processor has your information on file. Your card issuer knows this as well and some will send the information so that your subscription is not interrupted. For further details on why or how this is done, you would want to contact your card issuer.