You'll have to let us know what sort of MTA you're using. Qmail? Postfix? You mentioned Outlook is your mail reader and that's kind of the opposite end of where you should be working. You have to work on the server side and get that setup for sending and receiving mail using SSL. Only problem is, it doesn't work like that.
If you want to secure qmail (for example) with a SSL certificate, it's not going to work. You'd have to use an SSL tunnel and pipe your secure mail through another port. I'd really like to do this myself and I'm looking for good tutorials on how to do it, but I'm not finding much. I've looked at www.stunnel.org
but I'm having trouble following it myself.
Now on the other hand, if you're wanting to use a web-based email client like Squirrel mail or Webmail, you can SSL enable this using your OpenSSL certificate and configuring your web server to use SSL for this connection. I just finished doing that yesterday night, so I'm pretty familiar with how its done. I used Apache and got a authorized certificate free for 6 months from http://certs.ipsca.com.
The only problem with this is you'll have to depend on your users to use an HTML based email reader for all of their mail, otherwise it's not SSL enabled. POP3 and SMTP would need to be SSL enabled using STunnel or by some similar means.