exchange email collecting

I am running MS exchange 2000. There is a user who has a local account with our domain and a seperate account with another domain. when i sent email to that user from our domain to his outside domain it is collected to his local account by exchange. I would very much like this not to happen and instead be delievered to his external domain.

In his user profile in active directory, I have checked and his email address was set both was, but it does not seem to make a difference.

Any tips?

Thank you in advance.