If you enable TLS on your existing SMTP virtual server then it will affect existing users in that you will not be able to receive most of your email. You must use a different host name and IP address for TLS on Exchange 2003.
Better still, upgrade to a newer version of Exchange which can handle this within your existing MX records. Exchange 2003 is EOL.
You will find that a lot of the companies that require you to use TLS will NOT want to use a different host name for the traffic.