Bounce and error codes

Each page explains one bounce or error code and what to change. Start with the group that matches your bounce.

Last updated 25 Oct 2026

Not sure which kind you have? Hard bounce vs soft bounce sorts every code by who owns the fix.

A bounce with no code yet

What the opening words of the notice mean, and where to find the real reason below them.

Mail Delivery Subsystem did your message arrive? Message blocked what Gmail's bounce means Delivery Status Notification Failure what to do next Email bounced back what the reason line says Your message could not be delivered Outlook's NDR explained 550 permanent failure what it means and how to clear it Message not delivered what Gmail's bounce means and the fix Undelivered Mail Returned to Sender a Postfix bounce

SPF, DKIM or DMARC failed

The address is fine, and the receiver refused the message after checking your domain's DNS. You can fix these yourself.

550 5.7.515 why Outlook rejects your mail, and the fix 550 5.7.509 does not pass DMARC verification, fixed 550 5.7.26 Gmail blocked your unauthenticated email 550 5.7.1 rejected per DMARC policy causes and fix 550 5.7.23 SPF validation failed at Microsoft 365 dmarc=fail (p=REJECT) SPF and DKIM did not align SPF PermError too many DNS lookups, and how to fix it DKIM body hash did not verify what changed your mail

Wrong address or missing mailbox

Changing your DNS won't clear these. Fix the address you typed, or ask an admin at the other end.

550 5.4.1 Recipient address rejected who can clear it 550 5.1.1 the mailbox does not exist, and the fix 550 5.2.1 the mailbox exists and is not accepting mail

Spam, reputation or a rule at the recipient

Authentication passed and the receiver still refused the message. These take longer to clear, and the fix is still on your side.

421 4.7.28 Gmail is rate limiting your IP or domain 554 5.7.1 the receiving server refused your message 550 5.7.350 the remote server refused the message 550 5.7.520 the tenant blocked external forwarding

No bounce at all

The receiver accepted the message and then held it. You can only see that in your DMARC reports.

Email quarantined no bounce, and what a sender fixes Email delivery failed with no bounce where it went and why

Elsewhere on the site

SPF and DKIM by provider The records each platform needs, and where alignment breaks. Guides Setup, alignment, aggregate reports and the ten-lookup limit.

Or check a domain right now with the free DMARC, SPF and DKIM lookups.

Find a failing sender before it bounces mail

A sender that fails authentication shows up in the DMARC reports, whether or not its mail bounced. We read them, and on a paid plan we email you the day a new sender first fails.

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