Can’t reply to senders not in Contacts List in Outlook 2010

Hi

I recently installed Outlook 2010 and Windows 7 on my wife's computer, after upgrading her from Windows Mail and Vista. I imported her existing mail and contacts. The problem we now have is that when she tries to reply to an email received from someone not in her Contacts List, the attempt fails. She is able to click on the Reply button and type a reply message, but when she clicks on the Send button, a message pops up:

Check Names

MS Outlook does not recognise "xxxxx"

Select the Address to use

No suggestions

The only way to send the message appears to be to add the person to the Contacts list.

There are no problems in replying to a message received from someone in her Contacts list.

When I compare her messages to those in my installation of Outlook 2010 I note that I can right click on any name in a To: or From: box and can get the properties and hence the person's email address. With my wife's installation, when I right click on a name all I can see is Cut, Copy, Paste and Clear greyed out and Select in black and clickable. This may perhaps provide a clue to what the problem is.

Thanks to everyone in advance for their advice on solving this problem.

John