We have a customer who we are migrating from SBS 2003 to Server 2012 with Exchange 2010. All mailboxes and public folders have been migrated to Exchange 2010, and all inbound SMTP email is being delivered to Exchange 2010. So far so good.
Yesterday evening, I stopped all of the Exchange 2003 services on the SBS. I sent test emails in and checked that they had arrived using OWA and Outlook 2003 (I have a domain admin account).
This morning, the customer phoned to say that some user's Outlook is prompting for credentials.
As soon as I enable all Exchange 2003 services, the problem goes away!
I'd like to decommission Exchange 2003, so any help would be appreciated.