Provider-specific problems.
WHY IT HAPPENS
Each provider filters differently.
Mail acceptable to one may be filtered by another.
WHAT USUALLY CAUSES IT
Reputation with that specific provider Complaint rate from their users Authentication problems
WHAT TO CHECK
That SPF, DKIM and DMARC all pass Whether you are blacklisted Whether recipients at that provider marked you as spam
THE COMPLAINT POINT
A few complaints from one provider's users affects delivery to all of them.
WHAT TO DO
Ensure you only send to people who asked Make unsubscribing easy Remove inactive addresses
WHAT ELSE HELPS
Asking recipients to add you to their contacts Encouraging replies, which signal legitimacy
FOR MAJOR PROVIDERS
Some offer sender tools showing your reputation with them.
Worth using if you send volume.
WHAT NOT TO DO
Assume it is a filter error Keep sending at volume while delivery is poor
That worsens the reputation further.
