Ever updated a domain record, and found that your Outlook still caches the old record? I was hit with this last week while on vacation (everything happens on vacation). I had been emailing my boss, when our corporate office changed his record (actually deleted it), and made “live” his actual email account, rather than a forwarder account. I suddenly started getting bounce backs reporting the account didn’t exist any more.
It turns out outlook caches this in a .nk2 file in the user’s application settings directory (for those unsure, this is usually <default drive>:\Documents and Settings\<username>\Application Data\Microsoft\Outlook).
There are two solutions to a cached entry. Delete the above mentioned file (while outlook is closed), or, when you start to type the name, and the drop down appears to suggest the completion, you can hit the down arrow, and hit the “Del” key.