When I started Outlook, I will receive a prompt for NT domain credent

I ran a testing last week. After I disconnected all PDC and BDC server from domain ( exchange 5.5 server still existing in domain ), I replaced another PDC and BDC ( they both sync. with original PDC and BDC a week ago). I reconnect orginal servers to domain after finished the testing.

This week, starting from Mon., a number of users reported when they started Outlook, they will receive a prompt for NT domain credential. I have searched for a solution through internet but can’t find any helpful information. As the time goes by, a few user can check their email again but other still cannot.

Any professional can give me advice? Please help~