This is a two part process.  First check to see if the server supports TLS by doing the following:

• telnet mail.server.com 25
  220 mail.server.com ESMTP PostFix
• ehlo testing
  250 mail.server.com
  250-PIPELINING
  250-SIZE 10240000
  250-VRFY
  250-ETRN
  250-STARTTLS
• quit

Once you have determined that starttls is a supported command do this:

openssl s_client -starttls smtp -crlf -connect mail.server.com:25

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