I'm having problems sending encrypted mail from Zimbra to our SMTP relay
server specified by zimbraMtaRelayHost.

I've tried the settings in

Outgoing SMTP Authentication - Zimbra :: Wiki

and a bunch of other stuff. Enabling session logging on the SMTP server shows that
Zimbra just ignores the STARTTLS option entirely (there are no attempts to use it).
checktls.com says that the relay is set up well, and other servers can send mail to
that system without any problems.

We are *not* requiring SMTP AUTH; we just want TLS. Does anyone have any
ideas for our Zimbra 7.2.4 server? Even enabling smtp_tls_note_starttls_offer
doesn't log anything.

Thanks for any help!