When looking into PLONE-5 directories, I found that:
/opt/plone/buildout-cache/eggs/zope.sendmail-5.0-py3.5.egg/zope/sendmail/mailer.py
It is apparent that PLONE does not propose an SSL connection, since that file starts with
from threading import local
from ssl import SSLError
from smtplib import SMTP
There is no "from smtplib import SMTP_SSL"