If a non admin user @mentions another non admin user in the activity stream for the record, both the users are not able to see the email in sys_email table.
Any supported version
Script in table level read ACL for sys_email table checks for the user's access to live_notification table for @mentioned emails. If the script fails, it would restrict the user from seeing @mentioned emails in sys_email table.
Create table level ACL on live_notification table according to the requirement so that the user has access to live_notification table.