Knox 2.1 and below
The Samsung email client inside the Knox container is not syncing emails.
This issue may occur if a certificate stored in Knox Client Certificate Manager (CCM) is required to sync emails. When the Knox container is locked, Knox CCM is also locked. Because of this, the apps inside the container cannot access certificates stored in Knox CCM. The email app stops syncing messages since it cannot access the required certificate.
From Knox 2.1, you can whitelist applications to access certificates even when the Knox container is locked. This feature's availability depends on MDM implementation. If you are using Knox 2.1 or above, whitelist the application that you are having problems with.