Outlook 2007 Addresses Disappearing

Small corporate network with clients running Outlook 2007 through POP server (no Exchange).  I have several users, myself included, that have run into an issue with seemingly random contacts email addresses disappearing.  

An example:  I just went to send an email to someone here in the office.  I have sent this person email every day for 10 years.  Today I try to send the mail and it won't go.  So I delete him from the TO: line and click on the TO: button to open the address book.  I find his name in the list, it shows his display name, and in the email address column it's blank.  I open his contact, enter the address again, save it, and send the email with no problem.  

This has been happening with random contacts for about the past few months or so.  It is also happening to other users.  

One thing we may all have in common is we all sync outlook with an iphone.  other than that I cant figure out why the email address would just suddenly be gone.  Any thoughts greatly appreciated.