Symptom: emails are delivered but land in spam or get quarantined.
1. Confirm the cause, starting with authentication (the most common cause): verify SPF and DKIM records are still present with a DNS lookup tool like dns.email. Then check DMARC: it is not set up automatically during domain verification but is strongly recommended.
2. Open the email in the Resend dashboard and review Deliverability Insights. It runs best-practice checks and flags: links that do not match your sending domain, missing or invalid DMARC, missing plain-text version, no-reply sender addresses, large body size, and open/click tracking on sensitive emails.
3. Read the provider's own hint: Gmail shows a banner on spam messages explaining why (e.g. "similar to messages identified as spam" means content filters; "seems dangerous" means a flagged URL or content).
4. Check reputation: use Google Postmaster Tools for your domain and IP reputation with Gmail. New domains should be warmed up, starting at low volume and increasing over time.
5. Fix sending practices: use a dedicated address per email type, never send to purchased or non-opted-in lists, include a visible unsubscribe link in marketing mail, keep HTML simple (image-only emails trigger filters), and match link domains to your sender domain. Outlook exposes SCL and BCL scores in headers for its own placement.
6. Verify: send to seed inboxes at the affected provider and confirm inbox placement before resuming volume.