Im using v2.5 and how now started reinstalling as it was quicker & i was only testing.
Heres my life story if you fancy reading it again:
I had 1 HR admin account, subscribed to all notifications (notifications were being received) at this point there was only 1 HR admin.
I then changed the email address notifications were going to. That is when the notifications started to fail.
Added another HR admin account, subscribed to notifications. Notifications were then being sent to both addresses but the log still said ‘failed: no to header’.
So I added another HR admin account which was subscribed to receive notifications. They stopped being sent again with the error: Failed: no to header.
I have deleted accounts etc to no avail.