[ExchangeList] Error "Interface not registered"

My Outlook 2003 is behind ISA 2004 server. Every time I try to send an e-mail, I receive the error "Interface not registered". Has anyone seen this error before?
If I reply to some of my old e-mails, I do not receive  the error. It only happens when I typed in a new e-mail address.

Now people don't seem to receive my e-mails when I reply to their old e-mails. Anyone seen the error before and how to resolve it? All searches on Microsoft, google and some other areas on the Internet have turned out to be negative. Thanks.

Tee

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