Thanks for the update, yes I checked the referred document already.
In my case the scenario is different, the notification is currently in closed status.
Also, coming to user notification settings the email id is valid and the notification preference is set to "html mail with attachments".
If notification is closed, STATUS will be CLOSED and MAIL_STATUS will be null in wf_notification table
Its common behaviour
Please also query wf_local_roles where name = 'USER_NAME' to see if the email_address there is valid.
Run wfmlrdbg.sql < notification id> to see complete picture of the notification being sent.