I'm using letsencrypt in two ways: a docker stack using certbot directly, and another using traefik (which performs certificate management automatically, using lego). In both cases there's a setting for the registration email address.
Is that email address stored in the certificate, or elsewhere? And can anyone (other than letsencypt) discover that email address?
The correlation of email to account/domains only exists in two places:
1- LetsEncrypt's database, which associates an Email Address to an Account ID. LetsEncrypt staff can pull this info themselves. You can pull this info yourself by presenting LetsEncrypt with the current AccountKey, which happens under secure https connections. No third party can access this information from LetsEncrypt. Intercepting this information during an API call would require a MITM or other attack.
2- If LetsEncrypt sends an expiry email, that email may be relayed to you by trusted third parties. It is possible, but incredibly unlikely, that (i) LetsEncrypt's email service providers or (ii) your email service provider, are mining this information from the emails they relay and store.
Your email address is only used by LetsEncrypt to inform you of a pending expiration, or the like. No one sees it here or anywhere else that you haven't published it openly.
i can message you here, but your email is not published at all. Rest easy.