The notification on Galaxy Watch for a new email doesn't give the 'mark as read', 'view on phone' and 'delete' actions. After a lengthy investigation, it seems that the notification sent to the watch is malformed, causing the actions not to show.
I have reported this to Samsung (twice) but no sign of a fix.
I have written a workaround app which intercepts the notification from Samsung Email and replaces it with one that works.