Mailboxes selected for scheduled sync are automatically deselected and event log shows show Mailbox disabled in Active Directory.
This issue applies to 'Resource Forest' deployments. The environment is spread across 2 AD forests. User accounts reside in one AD forest, and the Exchange servers and mailboxes in another. This results in a mailbox that is linked to the user account in a separate forest.
Mailbox disabled in Active Directory
Folder Sync is deployed in the Exchange forest to be able to access the mailboxes. In this forest there is a linked mailbox and a disabled user account (a mailbox cannot exist without a user account). A linked mailbox is a mailbox associated with an external account. The resource forest scenario is an example of when you would want to associate a mailbox with an external account. In a resource forest scenario, user objects in the Exchange forest have mailboxes, but the user objects are disabled for logon. You must associate these disabled user accounts in the Exchange forest with enabled user objects in the external accounts forest.
Folder Sync sees the disabled user account but is not able to see the user account in the external accounts forest. This causes Folder Sync to automatically deselect the mailbox for scheduled sync and list the mailbox as disabled in Active Directory.
This issue has been corrected in Folder Sync version 1.10
The upgrade can be downloaded here: https://archive.atlassian.net/wiki/display/admin/Cloud+Archive+Downloads
Please follow the upgrade instructions here: https://archive.atlassian.net/wiki/display/admin/Folder+Sync+Upgrade+Guide
After upgrading please uncheck the deselect disabled mailboxes checkbox as shown below.