A little back ground first.
Our original setup was on premesis Exchange running on an SBS2008 server. The server crashed, we had to rebuild the domain, and the customer opted to go with Office 365. Since there was no DC or Exchange to migrate from we recreated all the email accounts in Office 365. The rebuilt DC is still an SBS2008 server though Exchange features are not being used. Also, we migrated user workstation profiles using Windows Easy Transfer, added new email profiles in Windows, and imported PST files to each user account.
The problem is that clients using Outlook 2007 are getting messages every 15 to 30 minutes (not just when outlook is opened) asking for logon credentials. The message will occasionally list the Office365 server, but most of the time it says "welcome back to remote.[oldserver].com" and have both username and password fields blank. You can't enter any credentials to satisfy it, only cancel will continue on, and mail will continue to function.
Outlook 2007 has SP3 installed on all workstations.
Any help would be appreciated.