Workflow notifications not being sent to mailbxes and in some cases to the worklist as well

Hi All,

The custom workflow is not able to send email notifications. In one instance we could to receive the notifications in approver worklist but not approver email box.
In other instance we would not be receiving any notifications in approver worklist as well as in approver email box. But the workflow status diagram showing it was notified to the respective approver in both the instances.
The mail and notification both are going to the user attached to SYSADMIN user.

Please through some light on what areas we could investigate to find the root cause.
Any help is appreciated.

Thanks,