Issue remains in Manager Desktop 16.3.37. The TLS handshake still fails. Tested with Zoho and Hotmail on both my Macs.
Out of curiosity, I’ve also tested in linux using Ubuntu 14.04/15.10 and the issue does not occur there. SSL SMTP happily connects, so this does seem more specific to Mono on OSX, as I presume linux is also using Mono.
Now for the plot twist.
I also downloaded the trial of Manager Server, installed the Mono Framework as required and started the server.
Connected to my Manager Server, attempted my tests again and it worked like a charm.
Mono Framework being installed has not had any affect on the desktop edition.
I’m not saying Mono is in the clear, but this issue appears to only affect Manager Desktop on OS X.