New user not receiving external emails

Hi,

I'm very frustrating with the Exchange Server 2010, recently I add new user into it as before. The new user internal sending / receiving works without any problem, and the new user can send external email too. But someone else from the external email sending mail to this new user, the sender just got:

554 5.7.1 No such user in domain

But the new user is there in AD or Exchange server.

I check a lot with google, but without any ideas about that. The setting seems to be fine, and we don't have any policy. All old user works without any problem.

Can someone give me some ideas? Thanks!