If someone leaves the company and then comes back after a period of time, you may receive a non-delivery report when trying to email them.

This is a common issue when someone leaves the organization and returns.  It is related to the Outlook address auto-complete cache.

  1. Inside a new Outlook email, type the email address in the To: field you wish to clear from the cache until Outlook displays it
  2. Press the down arrow key to select the email address and hit the delete button.
  3. All Done. That entry should now be gone from the Outlook Auto Complete cache

Then add recipients name from the Address Book (or just type it in).