Hi,
This looks like an with relation to TLS. There could be many reasons why this is failing but I would start by updating the email templates module to ensure the latest Java libraries are being used. For example, according to https://jira.atlassian.com/browse/FE-7294 this could be due to an outdated javamail library.
Have you recently introduced any new app store module which might have introduced conflicting Java libraries?
Are you able to troubleshoot this by using a different email provider such as GMail, MailGun etc.? Are you also getting the same error?