The main causes, in order.
AUTHENTICATION FAILURES
SPF, DKIM or DMARC missing or misconfigured.
The most common cause and the most fixable. Nothing else helps while these are wrong.
SENDING REPUTATION
Your domain and sending platform's history. Complaints, bounces and sending to invalid addresses all damage it.
LIST QUALITY
Sending to addresses that bounce or never engage signals a poor list.
CONTENT SIGNALS
Excessive capitals, multiple exclamation marks, urgency language, a single large image with no text, and certain words associated with scams.
ENGAGEMENT
Providers watch whether recipients open, reply and move messages out of spam. Low engagement over time means more filtering.
SENDING PATTERNS
A sudden large volume from a domain with no history looks like a compromised account.
Volume should grow gradually.
THE ORDER TO FIX
Authentication first. Then list quality. Then content. Then patterns.
Most delivery problems are resolved at the first step.
