What are Gmail “reason codes”?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
When Gmail rejects your email, it doesn't just say no. It says no with a code that tells you why. These are Gmail's enhanced status codes, and they're genuinely useful for diagnosing exactly what went wrong.
The format is three numbers separated by dots, embedded in the SMTP response. Here are the ones you'll actually encounter:
5.7.1. Generic policy rejection. Gmail decided not to accept the message but isn't being specific about why. Often a reputation problem.
5.7.26. Authentication failure. Your message doesn't have authentication information that meets Gmail's requirements. Check that your DMARC record is present and that your domain is aligned.
5.7.27. SPF problem. Your sending IP isn't listed in your domain's SPF record, or your SPF record has an error.
5.7.28. DKIM problem. The DKIM signature is missing, invalid, or not aligned with your From domain.
5.1.1. The recipient address doesn't exist. Hard bounce.
4.x.x codes. Temporary failure. Gmail is saying "try again later." Not a permanent rejection. Your sending infrastructure should queue and retry automatically.
You'll see these codes in your ESP's bounce log or in your mail server logs. If you're on a cloud ESP, look for the raw bounce message. Most platforms expose it. You can also check Google Postmaster Tools (free) for a broader view of how Gmail sees your domain over time. If you're hitting 5.7.26, 5.7.27, or 5.7.28, our free SPF checker and DMARC parser can help diagnose which authentication record is the problem.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
