My boss has the problem when sending mail to some staff "this e-mail message cannot be delivered because the e-mail address is no longer valid". The solution below fixed it before and again today. However, the problem returns. Is there anything else that can be done.
With Outlook 2010 open, click File (BackStage), and then click Options.
Click the Mail tab.
Under Send Messages, click Empty Auto-Complete List.
Outlook will generate a new nickname cache.