Order confirmations and notifications.
WHY THIS MATTERS COMMERCIALLY
A customer who receives no confirmation assumes the order failed.
They contact you, or dispute the payment.
WHAT MUST ARRIVE
Order received Payment confirmed Dispatch notification Refund confirmation
WHAT USUALLY GOES WRONG
The shop sends through the default server function The sender address is wrong Authentication is not configured for that sending route
WHAT TO DO
Configure SMTP for the shop Send from an address on your domain Ensure that route is authenticated
KEEPING IT SEPARATE FROM MARKETING
A marketing complaint should not affect order confirmations.
Consider separate services or subdomains for each.
WHAT TO TEST
Place a real order and confirm every email arrives at a major provider.
Not only at your own domain.
HOW OFTEN TO RETEST
After any change, and periodically.
WHAT TO MONITOR
Customers reporting they did not receive confirmations.
That is the signal.