Send email problem using Exch 2010

Hi,

I already installed Exch2010 with coexistance with Exch2003.

Outlook client is using Internet Email (pop & smtp) to send and retrieve email.

I already delete the SMTP Outound at Exch2003 ESM.

Most of the Outlook client I have already change the IP address to point to the new Exch 2010. Email can send and out internal & external.

My problem is:

There are some user (7 users) that if I set smtp address of Outlook to new Exch2010 IP address, it keep prompt for username/password

But most of the Outlook client, i set the IP to new Exch2010, seem fine everything.

Why does it happen? any idea?