I think I have narrowed down my troubles for a remote user to that of certificate problems.
I have found what I believe to be the latest cert I created in this directory, /opt/zimbra/ssl/zimbra/ca/newcerts/. I have downloaded the .pem, renamed it to .crt so that I can import into the Windows Certificate store. It imports successfully, and I verify that the expiration date matches that of the one displayed in the Admin Console.
However, when I try and send an email from within Outlook with my mail server setup as an IMAP connection, I see the following error in my logs:
I have disabled zimbraMtaTlsAuthOnly, and the account can send email fine. When it's set to TRUE I run into this problem, which is why I don't think Outlook is playing nicely with my certificates - even after I have imported them into Windows.
Sep 21 13:16:19 mail postfix/smtpd: lost connection after EHLO from (hostname removed) [ip removed]
I'm at a loss here.