The unsubscribe link in the notification was clicked accidentally. Since we use our agency's email address on about 100-200 websites, I need a way to reactivate this email address so that I don't miss a message about an expiring certificate in the future.
You can use "+" addressing by changing the email address on your ACME account(s) from youremail@whatever.com to youremail+something@whatever.com. The "new" email address (the email address with +etc. is considered a different email address from the email address without +etc.) applies to all certificates issued for an ACME account, past and future. You could also use a completely different email address instead with the same effect.