What version are you running?
2.5.13.1
What steps will reproduce the problem?

Configure email settings in admin panel to use email account with TLS ENABLED on mail server that uses a domain with no valid certificate.
Submit a review or do any other action that should trigger an email notification.
Check logs in the administration menu.

What is the expected output? What do you see instead?
I would expect emails to be sent. The actual result is that emails are not sent.
Please provide any additional information below.
Be advised, I tried using accounts on 2 servers: Office 365 paid plan with Exchange Online and custom domain (but no certificate), some 3rd party hosting mail server with other custom domain (but no certificate). I will spare you the long story but the bottom line is I cannot just buy a certificate. As an alternative I can use unsecure connection (TLS disabled) however, this will soon stop being an option for me too, as the company currently migrates service accounts to different servers that require TLS (but have no certificates).
See log (personal data anonymized):

 -   - root - Error sending e-mail notification with subject '<subject>' on behalf of '<name> <email>' to '<name> <email>,<name> <email>,<name> <email>'

Traceback (most recent call last):

  File "/usr/lib/python2.7/site-packages/reviewboard/notifications/email.py", line 634, in send_review_mail

    message.send()

  File "/usr/lib/python2.7/site-packages/django/core/mail/message.py", line 276, in send

    return self.get_connection(fail_silently).send_messages([self])

  File "/usr/lib/python2.7/site-packages/django/core/mail/backends/smtp.py", line 87, in send_messages

    new_conn_created = self.open()

  File "/usr/lib/python2.7/site-packages/django/core/mail/backends/smtp.py", line 51, in open

    self.connection.starttls()

  File "/usr/lib64/python2.7/smtplib.py", line 648, in starttls

    self.sock = ssl.wrap_socket(self.sock, keyfile, certfile)

  File "/usr/lib64/python2.7/ssl.py", line 936, in wrap_socket

    ciphers=ciphers)

  File "/usr/lib64/python2.7/ssl.py", line 611, in init

    self.do_handshake()

  File "/usr/lib64/python2.7/ssl.py", line 833, in do_handshake

    self._sslobj.do_handshake()

error: [Errno 0] Error