I used to send emails to unsecured SMTP servers using nc.exe utility (Windows port of NetCat) and now I need to send the same emails to a secured SMTP server (TLS). What additional parameters do I need to specify in nc.exe or body of the email message to enable secure communication?
With netcat, it's more-or-less impossible*.
You need a utilty which supports tls/ssl - you can try with snetcat or any other netcat clones.
*you have to do a ssl handshake first, exchange keys, encrypt, send, and then decrypt recieved data
The following article explains how to achieve this with the combination of stunnel/blat http://www.jeffkastner.com/2010/01/blat-stunnel-and-gmail