Email bounce codes, decoded.
37 SMTP error codes from Google and Microsoft, each matched word for word against the provider's own reference on : what the code means, whether it is a hard or a soft bounce, whether warming has anything to do with it, and the fix. Paste the code or a piece of the message.
37 of 37 verified codes.
- 550 5.7.1GmailThe user or domain that you are sending to (or from) has a policy that prohibits the email that you sent.Recipient policy · hard
- 550 5.7.1GmailThe IP you're using to send email is not authorized to send email directly to our servers.Reputation · hard
- 550 5.7.1GmailThis message is likely suspicious due to the very low reputation of the sending domain.Reputation · hard · warming helps
- 550 5.7.1MicrosoftUnable to relayConfiguration · hard
- 550 5.7.1MicrosoftDelivery not authorizedRecipient policy · hard
- 550 5.7.26GmailThis email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticat…Authentication · hard
- 421 4.7.26GmailThis email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate w…Authentication · soft
- 550 5.7.25GmailThis message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS ent…DNS · hard
- 421 4.7.0GmailThis message is suspicious due to the very low reputation of the sending domain.Reputation · soft · warming helps
- 421 4.7.28GmailGmail has detected an unusual rate of unsolicited email originating from your IP address.Reputation · soft · warming helps
- 421 4.7.0GmailTry again later, closing connection.Rate limit · soft
- 550 5.7.515Microsoft550; 5.7.515 Access denied, sending domain [SendingDomain] does not meet the required authentication level.Authentication · hard
- 550 5.7.509MicrosoftAccess denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of rej…Authentication · hard
- 550 5.7.511MicrosoftAccess denied, banned senderReputation · hard · warming helps
- 550 5.7.606-649MicrosoftAccess denied, banned sending IP [IP1.IP2.IP3.IP4]Reputation · hard · warming helps
- 550 5.4.1MicrosoftRecipient address rejected: Access deniedRecipient problem · hard
- 550 5.1.1GmailThe email account that you tried to reach does not exist.Recipient problem · hard
- 550 5.2.1GmailThe email account that you tried to reach is inactive.Recipient problem · hard
- 552 5.2.2GmailThe recipient's inbox is out of storage space and inactive.Recipient problem · hard
- 452 4.2.2GmailThe recipient's inbox is out of storage space.Recipient problem · soft
- 550 5.7.23MicrosoftThe message was rejected because of Sender Policy Framework violationAuthentication · hard
- 451 4.4.2GmailTimeout - closing connection.Transient · soft
- 452 4.5.3GmailYour message has too many recipients.Rate limit · soft
- 550 5.4.5GmailDaily user sending limit exceeded.Rate limit · hard
- 550 5.7.27GmailThis message was blocked because it didn't pass SPF authentication.Authentication · hard
- 550 5.7.30GmailThis message was blocked because it didn't pass DKIM authentication.Authentication · hard
- 550 5.7.40GmailYour message was blocked because the sending domain doesn't have a DMARC record or the DMARC record doesn't …Authentication · hard
- 421 4.7.32GmailYour email has been rate limited because the From: header (RFC5322) in this message isn't aligned with eithe…Authentication · soft
- 550 5.7.29GmailThis message was blocked because it wasn't sent over a TLS connection.Configuration · hard
- 550 5.7.28GmailThere is an unusual rate of unsolicited email originating from your IP address.Reputation · hard · warming helps
- 550 5.7.1GmailThis message is likely unsolicited email.Content · hard · warming helps
- 550 5.7.1GmailThis email has been rate limited.Rate limit · hard · warming helps
- 553 5.1.2GmailWe weren't able to find the recipient domain.DNS · hard
- 421 4.7.0GmailThis message is suspicious due to the very low reputation of the sending IP address.Reputation · soft · warming helps
- 550 5.7.233MicrosoftYour message can't be sent because your tenant exceeded its daily limit for sending email to external recipi…Rate limit · hard
- 550 5.7.236MicrosoftYour message can't be sent because your tenant has exceeded its daily limit for sending email to external re…Rate limit · hard
- 550 5.2.2MicrosoftSubmission quota exceededRate limit · hard
37 verified codes by cause class; green share: warming is part of the fix
Not yet in the decoder: 9 codes whose wording could not be matched on a provider page (Microsoft, Outlook.com, Yahoo). Yahoo's reference documents error categories rather than the bracketed codes senders see, and Outlook.com's troubleshooting page moved behind an application shell; they are added as they are verified.
A bounce is the receiver's verdict on one message. Where the verdict is about reputation, the durable fix is the one warming supplies; where it is about authentication, DNS or the list, warming is beside the point, and each page says which.
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