When it comes to mails (be it SMTP or IMAP), "TLS" usually means STARTTLS (start the session as plain text then switch to secure layer using a STARTTLS mechanism, which is more or less protocol-dependent).
"SSL" usually refers to a classic SSL session (with ssl/tls handshake from the begining).

smtp servers (for enduser submission) usually have STARTTLS support on their submission (587) port, while they only have regular "ssl" on their smtps (465) port.
Some admins decide to just disable the smtps port while others keep it enabled as it might help with some users behind silly firewalls.

As for OpenMailbox, both ports work for me when using s_client, and it reports TLSv1 in both cases.
I'd suggest you try it as well and see if result differs.
Maybe libtinymail doesn't enable TLSv1 in SSL sessions but only for STARTTLS sessions, which would be a bug