This is Exchange Server 2010
An external user sent an email to A, and CC'd B and C. All 3 users are on the same domain hosted on Exchange 2010.
A and C did not receive this email but B did.
I checked "Tracking Log Explorer" in the Toolbox from Exchange Management Console, it did not show the email arrived at A and C's mailbox, but it showed the email arrived at B's mailbox.
I also checked the antivirus software's log, it did not block anything related to the missing email.
What could be wrong?
Do you have an external spam filter that processes all incoming emails before being delivered to your exchange server? If you do I would look at those logs to see if the email arrived at your premises.
A few things you could check.