ot: accepting self certs into win pc?

few month ago, I've got a new Dovecot/Postfix server with self issued certificate (like the previous server), transferred users, all went well

EXCEPT for one user on Win/Outlook (or Outlook Express) who tells me his new PC 'doesn't want to accept certificate' (sorry, I'm short on exact details at this time)

I need to get it sorted out, I expect it 'should just work' like it did for other users, BUT, before I start looking, trying to 'educate myself' better

if any one has any pointers, dos or don't regarding win email clients with self certified server, pls point me that way

is using IE with www.dom.com/mycert.crt good point to start ? (after copying mycer.crt to web linked directory first?)

thanks, V