This is probably not of interest to any of you.  I'm just posting it here because it's a problem I frequently face at work and I want to remember where the link is to solve it.  
Here it is. 
  Basically, when I reformat a computer or move one of my users to a new computer, I migrate their Outlook .pst file that contains all their mail and contacts.  However, unless I follow the steps in 
this link, they cannot add addresses to emails from their Outlook contacts.