And of course, I just discovered Substack (specifically) doesn't allow email addresses from Kill the Newsletter. Another reason to dunk on Substack.

You may have to create an alias for your email address. If you're not familiar with creating an alias you may want to try one of these services:

https://simplelogin.io/
https://burnermail.io/
https://relay.firefox.com/
https://anonaddy.com/
https://altmails.com/
https://duckduckgo.com/email/