Let's Encrypt sends notifications about your SSL's and it can be an email like the following:
Your certificate (or certificates) for the names listed below will expire in 19 days (on 01 Apr 20 16:00 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
We recommend renewing certificates automatically when they have a third of their
total lifetime left. For Let's Encrypt's current 90-day certificates, that means
renewing 30 days before expiration. See
https://letsencrypt.org/docs/integration-guide/ for details.
For any questions or support, please visit https://community.letsencrypt.org/. Unfortunately, we can't provide support by email.
If you received an email like this, chances are it's because the site or server where this SSL was issued is no longer online. For example, if you add a site on server A, get an SSL, then move the site to server B, and get a new SSL (if the old one hasn't been migrated), you'll get an email about the certificate that was on server A.
One easy way to check this is to pull up the domain in question and confirm the dates match:
If the dates don't match, it means the SSL currently on your site is not the one that is actually expiring. You can probably disregard. If you're ever in doubt, contact support, and we would be happy to check.