Microsoft mail servers 550 5.4.300
Routing
The receiving service refused the route, connector, or relay path. The recipient may still be valid, so fix routing before making a suppression decision.
Decision
Review the full SMTP reply before retrying.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- Any SMTP stage
- Endpoint
- Inbound MX
Message expiredStart here
- 1
Check MX records, forwarding paths, and connector configuration for loops or relay denial.
- 2
Retry only after the routing path has been corrected.
Confirm the cause
- Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
- Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
- Test with one recipient before retrying bulk traffic.
App handling
- Route to manual review or a conservative fallback classification.
- Do not suppress the recipient from this rule unless a later bounce gives stronger recipient evidence.
- Limit automated retries until the provider-specific cause is understood.
Avoid
- Do not decide from the SMTP code alone.
- Do not suppress unless the provider-specific evidence points to an invalid recipient.
- Do not retry high-volume traffic before you understand the diagnostic text.
Common causes
- The provider-specific diagnostic text matches this class of failure.
- The same SMTP code may have another meaning when the provider, stage, or endpoint differs.
- Sender setup, recipient status, and provider policy should all be checked before taking irreversible action.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampsource IPconnectorTLS result