Deliverability is decided by the receiving provider, and several factors combine.
THE USUAL CAUSES
Missing or incorrect SPF, DKIM and DMARC records Sending from a Gmail or Yahoo address in the From field of your website forms, rather than an address on your own domain Your website using PHP mail() rather than authenticated SMTP A new domain with no sending history
Content that resembles marketing: heavy formatting, many links, aggressive subject lines
Recipients marking earlier messages as spam Sending to addresses that bounce, which signals a poor list
WHERE TO START
cPanel > Email Deliverability. It shows SPF and DKIM status for every domain and repairs them where we host the DNS. Fixing authentication resolves most cases.
THEN
Configure your website to send through SMTP with a mailbox on your domain. Send test messages to Gmail and Outlook addresses you control and read the headers. Ask recipients to mark your mail as not spam, which teaches their filter.
Reputation builds over weeks of ordinary sending. It cannot be bought or rushed.
