"The server you are connected to is using a security certificate that could not be verified" - Is not popping up in some Outlook Xp & 2003 clients

Outlook 2003 & XP does not popping up the below message in some PC's "The server you are connected to is using a security certificate that could not be verified."

We are using SMTP server with a security certificate enabled. but some of my outlook XP & 2003 client PC's not always popping the message to select to "Yes" Unless it select as yes the email will not go and stuck in the out box.

Some times its even popup first time when outlook starts and then again when the PC restarted it is not popping up when user tries to send an email.

This is behaving very unpredictable way. Outlook 2007 & 2010 clients are OK.

Any advice how to fix this issue

Roshan