Campaign emails not delivered but preview test email works fine

Hi ... I have a user who is not receiving email campaign items, yet I can send them a preview without problem (same email) so their own email is not blocking / filtering (unless the preview engine is different from the campaign engine).  The campaigns are being received fine by ~800 other users.  This is a key user in our organization, so I need to get this fixed.  Any help / suggestions on how to troubleshoot are much appreciated.