Resend emails landing in spam: authentication, links, and content checklist

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

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.

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.

Documented in Resend knowledge base: authentication issues are the most common cause of spam placement (SPF auto-set, DKIM at verification, DMARC recommended); Deliverability Insights flags link-domain mismatch, missing DMARC, missing plain text, no-reply senders, large body, tracking on sensitive emails; Gmail spam banners explain the reason; Google Postmaster Tools shows reputation; new domains need warming. Evidence: resend.com/docs/knowledge-base/why-are-my-emails-going-to-spam.