Question : Outlook user can replay to internal email, but not send new email

Hi everyone,
I am having a bit of a issue with a user.  The issue the user is having is that he can reply to internal emails but he can not send new emails to the address.  He can send emails outside there network, and he can recieve emails from inside and outside there netowork.   All the other users on his network can send emails between themselves, he is the only one with this issue.
They are running exchange server 2003 And SBS 2003.  User is using windows XP with Outlook 2000

Answer : Outlook user can replay to internal email, but not send new email

Is the user using a cached GAL?  If so, delete and download a new version.  Also,  delete his .nik or .nk2 file.  This would be in the C:\documents and settings\"user"\Application data\microsoft\outlook folder.  This would wipe the names in his "autocomplete ", but clear out any errors.