Hello, when trying to send emails via SSL (pop3 995 and smtp 465 or 587) I got this error: --- start --- the server you are connecting to is using certificate which cannot be validated. Certification chain has been processed, however, was terminated in root certificate, which is not trusted by the trust provider. --- end --- I use Lets Encrypt certificate on Postfix 3.3.0 and Ubuntu 18.04 here. On other independent servers I also use either Lets Encrypt on postfix 2.9.6/Debian wheezy or postfix 3.1.0/ubuntu16.04, and everything is in order = no errors in the same Live Mail.
Use Internet Search Engines with Code: ssl testing mail server Those tools check certificate E-Mail server is using. See if it is the one you installed. It should also show why it is not trusted, this may provide clues.
