For every user that will be synchronized, ensure that the minimum security access control is provided.
The 'Connector Service User' requires the following permissions as the minimum subset:
Access: Editor with “Delete documents”, “Create personal folders/views”, “Create shared folders/views”, “Replicate or copy documents”.
In the following example, the Connector Service User is "QA SyncUser" and the target mail file belongs to user "QA User 1":