[exim] Exim 4.43 as server and Outlook 2000 as client

Hi,

I'm hoping somebody can give me an idea on how to resolve this...

I've got Exim 4.43 running on a Linux machine as my MTA, with TLS/SSL
support and what I believe are called self-signed certificates. What I
would like to stop happening is that whenever I open up my Outlook 2000 (or
Outlook Express) I get a dialog which says:

"The server you are connected to is using a security certificate that could
not be verified.

A certificate chain processed, but terminated in a root certificate which is
not trusted by the trust provider.

Do you want to continue using this server? ... "

So, who is the "trust provider", and actually, how do I get Outlook to
accept the server's certificate, and stop presenting this dialog at every
startup?

Thank you very much in advance for all help!
Robert