How-to: let LetsEncrypt certificates being validated in MacOSX < 10.12 (old Mac):
- Download the ISRG Root X1 self-signed certificate from https://letsencrypt.org/certificates/
- Open the certificate app “Keychain Access” from Applications/Utilities
- Add (+) the ISRG Root X1 certificate above in “System”
- Go in File->Get Info to set its trust preferences for SSL to “Always trust”
Enjoy!













