The ACME protocol actually requires submission of a list of email addresses per account. CertSage (the ACME client that I authored and maintain) accepts one email address per line in a text box. An empty list is also allowed. This list is used for expiration emails for every certificate issued via that account, which includes certificates already issued prior to updating the list. The capabilities of your ACME client may or may not allow submission of a list of email addresses or updating your email address(es) at all. You can update your email addresses by simply installing CertSage somewhere, opening it in your browser, copying your ACME account key from your existing ACME client into account.key in your CertSage directory, then using the Update Contact Information button in CertSage.