Thanks Nick for taking the time to answer. I really appreciate it.

My Email Server is an ISP hosted MS Exchange setup and for the longest time they only supported SSL and not STARTTLS, no idea why. However, made curious by your comment I just checked and lo and behold, once I specified protocol=smtp to coerce the server into listening for plain text at first and only then switching to STARTTLS, it worked like a charm with port 587.

Thanks also for pointing out the CentOS 7 stunnel 5.40 package on pkgs.org. How compatible are CentOS<>ClearOS really when it comes to rpms? For instance, is a CentOS "7" package an indication that this should work with COS7, etc.?