Delivery reaching the wrong folder.
WHAT TO CHECK
SPF, DKIM and DMARC, in cPanel Email Deliverability Blacklist status Whether the sending source is authenticated
THE AUTHENTICATION POINT
Every source that sends as your domain needs authenticating separately.
Your ordinary mail may be fine while your site's mail is not.
IF ONLY ONE PROVIDER FILTERS IT
Reputation with that provider, or complaints from their users.
IF IT STARTED SUDDENLY
Check for blacklisting Check whether anything changed in DNS Check whether spam was sent from your account
THAT LAST ONE
A compromise sending spam gets you blacklisted.
Check for it.
FOR FORM AND ORDER EMAILS
Configure SMTP so the site sends through an authenticated account.
Send from your own domain, never the visitor's address.
WHAT ELSE AFFECTS IT
Sending to people who did not ask A difficult unsubscribe Old or purchased lists
WHAT NOT TO ASSUME
That the content is the problem. It rarely is.
