I have exactly the same problem.
On my developement-machine (vagrant with ubuntu/trusty32, nginx, php7, CakePHP 3.3.0) I am having no problems to connect the smtp-server an send mails.
On the production-server (a cloud-based service, php7) I get the described error.
I found this post describing a similar behaviour. But my SSL-skills are not very good. Maybe this helps to investigate the cause of the error.