Emails to three users are bouncing but not to others

I am working for a small organization with maybe 200 mailboxes in their Exchange 2003.  They use constant contact to send out bulk emails to their people as well as to people who support their organization.  Every time they send out a bulk email two or three of their internal users do not get the emails.  The odd thing is that out of 20 or so of their internal people who are on this list, only these three do not get the email.

They contacted constant contact to ask them for help and the response from CC was sending them the NDR which is:

status=bounced (host mail.ajrsem.org[108.46.205.66] said: 550 5.7.1 Requested action not taken: message refused (in reply to end of DATA command))

Again, this only happens for three of the names on the list, so I think it cannot have anything to do with their domain.  I would think it has to do with the specific people.  I have tried whitlisting whatever I can, but with Exchange 2003 it is limited.

They are also using Sonicwall Spam filtering.  This was added recently, the problem was appearing before they started using this and the same three people are not getting emails.  Since there was no major change, I would think that the problem is not with the Sonicwall.

I am being put under pressure to figure this out especially since one of the three email addresses is for the head of this organization.  Can anyone help?
pgoldwasserAsked:
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Simon Butler (Sembee)ConsultantCommented:
The NDR is pretty clear - Exchange has refused it. That could be they have flagged the sender as spam in their Outlook for example, or a third party tool is causing the rejection. If you have other users in the org who are able to accept email then it would tend to point to spam filtering at the client level.

Simon.
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pgoldwasserAuthor Commented:
But would the outlook SPAM filter cause a NDR?  I thought that the outlook SPAM filtering just put the emails into the junk mail folder.  There is no third party tool here.  We are NOW using Sonicwall spam filtering, but the problem existed the same way before we started using that, so it must be independent of the Sonicwall.  If it was the exchange server, then why only those three people.  I will check their outlook settings to be sure.
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Simon Butler (Sembee)ConsultantCommented:
The Outlook spam filter integrates with Exchange. If a user flags a message then it is blocked by Exchange - it isn't just a client only solution.

Simon.
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