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.