The most common complaint.
CHECK AUTHENTICATION FIRST
Send to a Gmail address and view the original. It shows SPF, DKIM and DMARC results explicitly.
If any fails, that is the cause and nothing else matters until it is fixed.
THE COMMON AUTHENTICATION ERRORS
Two SPF records, which invalidates both A sending platform added without updating SPF A DKIM record copied incompletely Records added in cPanel when DNS is at Cloudflare
CHECK BLACKLISTS
Your domain and sending address. A listing explains sudden filtering.
CHECK LIST QUALITY
Sending to addresses that bounce or never open damages reputation over time.
Remove hard bounces and long-inactive subscribers.
CHECK CONTENT
Excessive capitals, urgency language, a message that is one large image, many links to unfamiliar domains.
Secondary to authentication, and worth reviewing.
CHECK SENDING PATTERNS
A sudden large volume from a domain with no history looks suspicious.
THE ORDER TO WORK
Authentication, blacklists, list quality, content. In that order.
Most cases resolve at the first step.
