550 5.7.26 📧 Email / SMTP

550 5.7.26 This mail has been blocked because the sender is unauthenticated (SPF/DKIM/DMARC)

Gmail/Outlook rejected the message because SPF and DKIM didn’t pass for the sender’s domain (DMARC failure).

Seen on: SMTP

Meaning

Since 2024 Gmail and Yahoo require bulk senders to authenticate. Mail sent from a server or service not listed in SPF and without a DKIM signature for your domain is rejected. Sending “From: you@gmail.com” via another server also fails.

Common causes

  • Sending server/service not included in the domain’s SPF record
  • No DKIM signature (or wrong selector)
  • DMARC policy p=reject/quarantine with misaligned From domain
  • Sending as a domain you don’t control

⚡ Quick fix

  1. Add the provider to SPF (include:…)
  2. Set up DKIM for your sending service
  3. Publish DMARC and align the From domain with SPF/DKIM

Detailed fix by platform

Shell

  1. bash
    dig +short TXT example.com | grep spf
dig +short TXT default._domainkey.example.com
dig +short TXT _dmarc.example.com

How to diagnose

  1. Headers — Authentication-Results: spf=?, dkim=?, dmarc=?
  2. DNS — SPF/DKIM/DMARC records

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

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