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.

Find an error code or provider

27 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