Why does this window "Authentication Required" appear when an external recipient opens an email sent from our exchange 2003 server?

Our CEO sent an email and the recipient, upon opening the email, was given a pop-up window that says "Authentication Required".  I've seen this occasionally with internal mail but not with outgoing mail.  We also have "Exclaimer" software running on our system.  The attached pdf shows the window...
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This does not look a normal behaviour of Exchange - seems some thing else is doing this - some other third-party utility.

If you have that Exclaimer software, my suggestion is that you shut that service down / remove it over a weekend and test if your situation improves.


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iteched1Author Commented:
I've been pulled on other projects so I'll close this question for now.  Thanks.
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