STARTTLS failed in version 4.7
This sounds like it's the same problem described here.

While the solutions there suggest hacks to the code, most likely you're either a) connecting to a mail server that has a self-signed certificate, or b) your CiviCRM is installed on a server with an outdated list of CAs.  Fixing the root problem is preferable.  Let's Encrypt issues free SSL certificates in case your problem is the former one.