Sendmail TLS and Certificate?

We are running HP-UX 11v1 and are about to upgrade sendmail to 8.13.3 to allow support for TLS. Enabling TLS seems pretty straightforward, but I'm wondering if an SSL certificate is required for this. Our MS Exchange server does use a certificate. Do I need to arrange for a public certificate to be installed to take full advantage of TLS?
Thanks,
Joe

Anyone?

I think any certificate will work, technically. It depends on the mail client if it accepts it. Usually the client will prompt the user with a 'do you accept?'.

Thanks for the reply. This will be only for server-to-server communications, no clients will connect to the Unix box. Does that make a difference? Can you point me to any documentation on this?
Thanks,
Joe