Symptom: delivered events on the webhook, but recipients find the mail in spam.
Cause candidates, in order of likelihood:
1. Missing or broken sender authentication. Confirm: check domain auth status in Settings > Sender Authentication. If SPF/DKIM/DMARC are not all validating, receiving servers distrust the mail.
2. Bad engagement. Confirm: high spamreport event counts and low opens relative to volume. Inbox providers demote senders whose mail is ignored or marked as spam.
3. Content and list problems. Confirm: spammy subject lines, no physical mailing address in the footer, no working opt-out, or old purchased lists.
Fix:
1. Complete domain authentication first. It is the highest-leverage single fix.
2. Put your company name in the subject area or header identity, include the physical mailing address and phone in the footer, and give every recipient a clear opt-out. These are the doc's explicit recommendations.
3. Stop mailing unengaged addresses. Continuing to mail people who never open trains the filters against you.
4. Honor spamreport events immediately: suppress the reporter and do not mail them again from any list.
Verification: watch spamreport rates per campaign after the fix, and seed-test to major providers. A falling spamreport rate plus rising opens is the signal.