Obscure exceptions when TLS cert on SMTP server is untrusted

[matrixbot]

This issue has been migrated from #9566.

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I think this is happening because my SMTP server is presenting a self-signed cert (and I can't stop synapse trying to STARTTLS, per matrix-org/synapse#8046)

2021-03-09 10:16:38,776 - synapse.push.emailpusher - 147 - ERROR - emailpush.process-15454 - Exception processing notifs
Capture point (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/synapse/src/synapse/app/homeserver.py", line 507, in <module>
    main()
  File "/home/synapse/src/synapse/app/homeserver.py", line 503, in main
    run(hs)
  File "/home/synapse/src/synapse/app/homeserver.py", line 494, in run
    logger=logger,
  File "/home/synapse/src/synapse/app/_base.py", line 135, in start_reactor
    run()
  File "/home/synapse/src/synapse/app/_base.py", line 119, in run
    run_command()
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/base.py", line 1423, in run
    self.mainLoop()
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/base.py", line 1433, in mainLoop
    reactorBaseSelf.runUntilCurrent()
  File "/home/synapse/src/synapse/metrics/__init__.py", line 548, in f
    ret = func(*args, **kwargs)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/base.py", line 999, in runUntilCurrent
    call.func(*call.args, **call.kw)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/tcp.py", line 507, in connectionLost
    self.connector.connectionLost(reason)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/base.py", line 1312, in connectionLost
    self.factory.clientConnectionLost(self, reason)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/mail/smtp.py", line 1935, in clientConnectionLost
    self._processConnectionError(connector, err)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/mail/smtp.py", line 1952, in _processConnectionError
    self.result.errback(err.value)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/defer.py", line 517, in errback
    self._startRunCallbacks(fail)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/defer.py", line 580, in _startRunCallbacks
    self._runCallbacks()
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/defer.py", line 662, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/defer.py", line 1511, in gotResult
    current_context.run(_inlineCallbacks, r, g, status)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/internet/defer.py", line 1443, in _inlineCallbacks
    result = current_context.run(result.throwExceptionIntoGenerator, g)
  File "/home/synapse/env/lib/python3.7/site-packages/twisted/python/failure.py", line 500, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
Traceback (most recent call last):
  File "/home/synapse/src/synapse/push/emailpusher.py", line 145, in _process
    await self._unsafe_process()
  File "/home/synapse/src/synapse/push/emailpusher.py", line 195, in _unsafe_process
    await self.send_notification(unprocessed, reason)
  File "/home/synapse/src/synapse/push/emailpusher.py", line 312, in send_notification
    self.app_id, self.user_id, self.email, push_actions, reason
  File "/home/synapse/src/synapse/push/mailer.py", line 308, in send_notification_mail
    await self.send_email(email_address, summary_text, template_vars)
  File "/home/synapse/src/synapse/push/mailer.py", line 360, in send_email
    requireTransportSecurity=self.hs.config.require_transport_security,
twisted.internet.error.ConnectionAborted: Connection was aborted locally using ITCPTransport.abortConnection.