Mailboxes in one domain are connecting to collaboration AD stub objects in subscribing domain.
We have users\mailboxes on the publishing domain that when they open outlook they are being prompted for credentials as outlook is trying to connect to the AD stub objects (coinciding AD objects) in the subscribing domain. There are 2 way trusts between the domains.
Exchange Auto discovery is seeing the stub objects in the subscribing side as the actual users objects when the actual objects are on the publishing side.
Prevent Auto Discovery from Connecting to the stub objects on the subscribing side.