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?