tl;dr: mail config works, but on trying to send mail synapse gets rejected by
postfix for using a too-old version of tls, as apparently tls in twisted (the
python library used for mail in synapse) is just hardcoded to v1, which our
postfix rejects.
```
postfix/smtpd[9737]: warning: TLS library problem: error:14209102:SSL routines:tls_early_post_process_client_hello:unsupported protocol:ssl/statem/statem_srvr.c:1685:
synapse[9211]: synapse.handlers.identity: [POST-41] Error sending threepid validation email to stuebinm@hacc.space
Traceback (most recent call last):
File "/nix/store/55mh6w2ark2blrbkyq0d1jjg9alb1dw5-matrix-synapse-1.29.0/lib/python3.8/site-packages/synapse/handlers/identity.py", line 382, in send_threepid_validation
await send_email_func(email_address, token, client_secret, session_id)
File "/nix/store/55mh6w2ark2blrbkyq0d1jjg9alb1dw5-matrix-synapse-1.29.0/lib/python3.8/site-packages/synapse/push/mailer.py", line 207, in send_add_threepid_mail
await self.send_email(
File "/nix/store/55mh6w2ark2blrbkyq0d1jjg9alb1dw5-matrix-synapse-1.29.0/lib/python3.8/site-packages/synapse/push/mailer.py", line 349, in send_email
await make_deferred_yieldable(
twisted.mail._except.SMTPConnectError: Unable to connect to server.
```
This is a known issue [1], which should be fixed in the current version of twisted,
which will be in the next version of synapse.
[1] https://github.com/matrix-org/synapse/issues/6211
