Hi Ben,
CAUSE
This issue occurs when you are using Microsoft Exchange Server as your mail system and when one of the contact mailboxes has been deleted from the Exchange server.
RESOLUTION
To resolve this issue, use the global address list to send the e-mail message. If the user no longer exists, delete the contact for the user. Alternatively, get the new e-mail address from the intended recipient to add to your Contacts folder.
If the message was sent to a group of contacts in a personal distribution list, open the distribution list in the Contacts folder, and then click Update Now. The e-mail address will be replaced with the display address of the user as in the following example:
o/=Contoso/ou=First Administrative Group/cn=Recipients/cn=Test User
When you add a contact that is also in the global address book, Outlook resolves the e-mail address to an Exchange Server address. If you remove the mailbox from the Exchange Server server, the address in the Contact list is not updated.
To check the e-mail address properties, follow these steps, as appropriate for the version of Outlook that you are running.
Microsoft Office Outlook 2003 and earlier versions of Outlook
1. Open a contact record.
2. Right-click the e-mail address, and then click Properties.
Microsoft Office Outlook 2007
1. On the Standard toolbar, click the Address Book icon.
2. In the Address Book list, click the address book that contains the name about which you want information.
3. In the Search box, type the name that you want.
4. In the list, right-click the name, and then click Properties.
For the problem contact, everything on the General tab but the Display Name is blank.
Microsoft Office Outlook 2010
1. On the Home Page, click the Address Book icon.
2. In the Address Book list, click the address book that contains the name about which you want information.
3. In the Search box, type the name that you want.
4. In the list, right-click the name, and then click Properties.