Failure intelligence
Email errors are clues. Use the whole clue.
Search provider responses and authentication failures, then move from the generic code to a diagnosis based on the complete bounce, message evidence and domain state.
66 diagnostic references
| Provider | Code | Meaning |
|---|---|---|
| gmail | 550 5.7.26 | Gmail 550 5.7.26 — Unauthenticated Email |
| gmail | 550 5.7.1 | Gmail 550 5.7.1 — Policy Rejection |
| outlook | 550 5.7.515 | Outlook 550 5.7.515 — Authentication Level Rejection |
| exchange-online | 550 5.7.703 | Exchange Online 550 5.7.703 — Recipient or Domain Blocked by Tenant Policy |
| exchange-online | 550 5.7.705 | Exchange Online 550 5.7.705 — Tenant Exceeded Threshold |
| exchange-online | 550 5.7.708 | Exchange Online 550 5.7.708 — Traffic Not Accepted from This IP |
| exchange-online | 550 5.7.750 | Exchange Online 550 5.7.750 — Unregistered Sending Domain |
| exchange-online | 550 5.1.8 | Exchange Online 550 5.1.8 — Bad Outbound Sender |
| exchange-online | 550 5.4.1 | Exchange Online 550 5.4.1 — Recipient Address Rejected |
| yahoo | 421.temporary | Yahoo 421 — Temporary Delivery Deferral |
| yahoo | 553.554.permanent | Yahoo 553 / 554 — Permanent Delivery Failure |
| gmail | 421 4.7.28 | Gmail 421 4.7.28 — Unusual Sending Rate |
| exchange-online | 550 5.7.23 | Exchange Online 550 5.7.23 — SPF Violation |
| exchange-online | 550 5.7.25 | Exchange Online 550 5.7.25 — IPv6 Reverse DNS Required |
| exchange-online | 550 5.7.57 | Exchange Online 550 5.7.57 — Client Not Authenticated During MAIL FROM |
| exchange-online | 550 5.7.64 | Exchange Online 550 5.7.64 — TenantAttribution Relay Access Denied |
| exchange-online | 550 5.7.321 | Exchange Online 550 5.7.321 — STARTTLS Not Supported |
| exchange-online | 550 5.7.322 | Exchange Online 550 5.7.322 — Destination Certificate Expired |
| exchange-online | 550 5.7.367 | Exchange Online 550 5.7.367 — Forwarded or Relayed Message Authentication Failure |
| exchange-online | 550 5.7.509 | Exchange Online 550 5.7.509 — DMARC Reject Failure |
| exchange-online | 550 5.4.14 | Exchange Online 550 5.4.14 — Routing Loop Detected |
| exchange-online | 550 5.4.8 | Exchange Online 550 5.4.8 — MX Failed MTA-STS Validation |
| exchange-online | 550 5.1.10 | Exchange Online 550 5.1.10 — Recipient Not Found |
| exchange-online | 550 5.1.20 | Exchange Online 550 5.1.20 — Multiple From Addresses Without Sender |
| exchange-online | 550 5.7.506 | Exchange Online 550 5.7.506 — Bad HELO Identity |
| generic | spf.permerror | SPF PermError — Too Many DNS Lookups or Invalid Evaluation |
| generic | dmarc.fail | DMARC Fail — Authentication Did Not Align |
| exchange-online | 550 5.7.1 relay | Exchange Online 550 5.7.1 — Unable to Relay / Not Authorized to Send |
| exchange-online | 530 5.7.1 | Exchange Online 530 5.7.1 — Client Was Not Authenticated |
| exchange-online | 550 5.4.310 | Exchange Online 550 5.4.310 — DNS Domain Does Not Exist |
| exchange-online | 550 5.4.316 | Exchange Online 550 5.4.316 — Message Expired, Connection Refused |
| exchange-online | 550 5.4.300 | Exchange Online 550 5.4.300 — Message Expired |
| exchange-online | 550 5.1.1 | Exchange Online 550 5.1.1 — Recipient Not Found (RESOLVER.ADR.RecipNotFound) |
| exchange-online | 550 5.2.2 | Exchange Online 550 5.2.2 — Mailbox Full |
| exchange-online | 550 5.2.1 | Exchange Online 550 5.2.1 — Mailbox Cannot Be Accessed |
| exchange-online | 554 5.4.6 | Exchange 554 5.4.6 — Hop Count Exceeded, Possible Mail Loop |
| exchange-online | 550 5.7.133 | Exchange Online 550 5.7.133 — Sender Not Authenticated for Group |
| exchange-online | 550 5.7.134 | Exchange Online 550 5.7.134 — Group Does Not Accept External Senders |
| exchange-online | 451 4.7.500 | Exchange Online 451 4.7.500 — Server Busy, Try Again Later |
| exchange-online | 451 4.7.650 | Exchange Online 451 4.7.650 — Temporarily Rate Limited Due to IP Reputation |
| outlook | 550 5.7.1 S3150 | Outlook.com 550 5.7.1 — Network on Block List (S3150) |
| outlook | 550 SC-001 | Outlook.com 550 SC-001 — Mail Rejected for Policy Reasons |
| outlook | 550 SC-002 | Outlook.com 550 SC-002 — Suspicious Sending Pattern |
| outlook | 550 SC-003 | Outlook.com 550 SC-003 — Open Relay or Malware Host |
| outlook | 550 SC-004 | Outlook.com 550 SC-004 — Reported as Spam by Users |
| outlook | 550 DY-001 | Outlook.com 550 DY-001 / DY-002 — Dynamic IP Address Rejected |
| outlook | 550 OU-001 | Outlook.com 550 OU-001 / OU-002 — Content Rejected |
| outlook | 421 RP-001 | Outlook.com 421 RP-001 / RP-002 / RP-003 — Rate Limited |
| gmail | 421 4.7.0 | Gmail 421 4.7.0 — Unusual Rate of Unsolicited Mail / Try Again Later |
| gmail | 550 5.7.25 | Gmail 550 5.7.25 — No PTR Record for Sending IP |
| gmail | 550 5.4.5 | Gmail 550 5.4.5 — Daily Sending Limit Exceeded |
| gmail | 552 5.7.0 | Gmail 552 5.7.0 — Message Blocked for Security Reasons (Attachment) |
| gmail | 550 5.1.1 | Gmail 550 5.1.1 — The Email Account Does Not Exist |
| gmail | 452 4.2.2 | Gmail 452 4.2.2 — Recipient Over Quota |
| gmail | 550 5.2.1 | Gmail 550 5.2.1 — Account Disabled or Receiving Rate Exceeded |
| yahoo | 421 TSS04 | Yahoo 421 4.7.0 [TSS04] — Temporarily Deferred Due to Volume or Complaints |
| yahoo | 554 5.7.9 | Yahoo 554 5.7.9 — Message Not Accepted for Policy Reasons (DMARC) |
| yahoo | 553 BL21 | Yahoo 553 5.7.1 [BL21] / 5.7.2 [TSS09] — Connection Refused, IP Listed |
| generic | 554 5.7.1 relay | 554 5.7.1 Relay Access Denied — Postfix and Other MTAs |
| generic | 550 5.1.1 | 550 5.1.1 User Unknown — Recipient Address Rejected |
| generic | 450 4.1.8 | 450 4.1.8 Sender Address Rejected: Domain Not Found |
| generic | 451 greylist | 451 4.7.1 Greylisted — Please Try Again Later |
| generic | 554 HELO | 554 5.7.1 HELO Command Rejected — Need Fully-Qualified Hostname |
| generic | 553 sender | 553 5.7.1 Sender Address Rejected: Not Owned by User |
| generic | SPF softfail | SPF Softfail — Received-SPF: softfail |
| generic | DKIM fail | DKIM Fail — Signature Verification Failed / Body Hash Did Not Verify |
No matching reference is listed yet. Paste the complete response into Mail Failure Doctor for evidence-based classification.