The receiving server refused the message on content or reputation grounds.
READ THE BOUNCE PROPERLY
It usually names the reason, and often includes a reference number or a link to the blocking organisation's page. That is the fastest route to the actual cause.
THE COMMON REASONS
SPF, DKIM or DMARC failing. Check cPanel > Email Deliverability and repair. The sending address is on a blacklist. Look it up and follow that list's delisting process. The From address is not on your own domain, which fails authentication.
Content resembling marketing: many links, image-heavy, aggressive subject lines.
Sending through PHP mail rather than authenticated SMTP, where the message came from your website.
WHAT TO DO FIRST
Fix authentication. It resolves the majority of these, and nothing else you do will help while SPF and DKIM are wrong.
IF IT AFFECTS ONE RECIPIENT ONLY
Their filter or their server policy. Ask them to whitelist you, or contact them another way.
IF IT AFFECTS EVERYONE
A reputation or authentication problem at your end. Work through the deliverability articles.
