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How to install Let's Encrypt SSL certificate

Make sure your site is available in all its aliases, e.g: www.yourdomain.tld

All DNS A / AAAA records of the site and aliases must point to the server.

Then inside ISPConfig web hosting control panel perform these tasks:
  1. Go to Sites module
  2. Click on the site you want the certificate
  3. Check the Let's Encrypt SSL field
  4. Click on Save button
You can test cypher strength and quality using SSL Server Test.

You may consider to add a DNS CAA record.