Small office network using MS Exchange 1 sender's email content can't be seen by just 1 user - why?

ok, I'll take a crack at this..
while I know of nothing that will block 1 exchange user from another, when I see such things happen I immediately think that there's a problem with internet explorer if the email comes in and nothing is viewable,
if the email doesn't even come through, I think spam filter.
since you say that the content can't be viewed, I'm leaning toward internet explorer and some combination of security vs stationary or signature problem.
is the content viewable when emails are sent as text?

We use a CRM package - Sage ACT - and the problem only seems related to emails sent from this product via Outlook. Since this Sage product will be upgraded soon we will live with the problem until then. Thanks for your response.