Almost always authentication. Set all three of these records and most deliverability problems disappear.
SPF is a TXT record listing who may send for your domain:v=spf1 include:flashnests.com ~all
DKIM is enabled in cPanel under Email Deliverability. It signs your outgoing mail cryptographically.
DMARC is a TXT record at _dmarc.yourdomain.com telling receivers what to do with failures:v=DMARC1; p=quarantine; rua=mailto:you@yourdomain.com
Start DMARC at p=none, read the reports for a fortnight, then move to quarantine and finally reject. Going straight to reject can silently drop legitimate mail.
Beyond authentication: avoid link shorteners, never send to purchased lists, and put a working unsubscribe link in every bulk message.
