TL;DR: Spam placement is usually authentication first, content second. Check SPF, DKIM, and DMARC with a DNS lookup, then open Deliverability Insights on the email in the dashboard: it flags mismatched link domains, missing plain text, and no-reply senders. 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. Open the email in the Resend dashboard and review Deliverability Insights.
The fix
TL;DR: Spam placement is usually authentication first, content second. Check SPF, DKIM, and DMARC with a DNS lookup, then open Deliverability Insights on the email in the dashboard: it flags mismatched link domains, missing plain text, and no-reply senders. 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. Open the email in the Resend dashboard and review Deliverability Insights.
The error
emails are delivered but land in spam or get quarantined.
The fix
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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.
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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.
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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).
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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.
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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.
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Verify: send to seed inboxes at the affected provider and confirm inbox placement before resuming volume.
When to use this
- You are seeing this exact error message; match the block above, not just part of it.
- The failing call matches the scenario in the title: Resend emails landing in spam.
- You want the fastest verified fix before digging through logs.
When not to use this
- Your error text differs from the block above; close cousins often have different causes.
- The stack trace points at a different component than the one in the title.
- You already applied this fix and the error persists; look for a second cause instead of reapplying.
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