Error sending Notification


New Member
Hi All.
I just start my store and i know less about WP things, i have problem in my Notifications email...
Purchase Notification, Contact Us, Order Shipped Notification all not work
" Error. invalid API key or sender domain used in API call. Hence authentication failed"
this notification come up in my screen.

I use Add On Abandon Cart.. its work.. all abandon cart notification sent .

What should i do?