Migrated to a new exchange hosted server

Hi,

We're migrated from a 2007 hosted Exchange service to a new provider offering the same service, however since the migration when we have outlook opened we keep getting a popup window appear asking for a username and password, however when we look at the exchange server address in the popup window, it's trying to look at our old exchange server.

Also in the windows credential manger Win7, we delete the old exchange saved passwords, but upon restarting the machine the login details re-appear again?

These machines are on group policy within our domain.

I need to stop the popup window appearing in outlook at the end users don't want to keep pressing cancel.

any ideas?