Microsoft mail servers 550 5.7.25
DNS configuration
The provider could not validate sender DNS cleanly. Check the sending domain records and wait for propagation after changes.
Decision
Do not retry unchanged. Fix the sender setup first.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- Connection
- Endpoint
- Inbound MX
IPv6reverse DNSStart here
- 1
Check DNS records for the sending domain, especially reverse DNS, SPF, DKIM, and DMARC records.
- 2
Wait for DNS propagation if you changed records recently, then retry the message.
Confirm the cause
- Check DNS records for the sending domain, including SPF, DKIM, DMARC, MX, and reverse DNS where relevant.
- Query DNS from more than one resolver if records changed recently, then allow propagation time.
- Retry a small test after DNS is visible, not while old records are still being served.
App handling
- Treat this as a sender setup incident, not a bad recipient.
- Hold similar mail for the same sender domain or provider until the fix is verified.
- Retry after a fresh single-recipient test succeeds.
Avoid
- Do not retry unchanged in a loop.
- Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
- Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
- Reverse DNS is missing or does not match the sending IP.
- Sender DNS records are missing, malformed, or not propagated yet.
- Recipient-side lookup failed while checking sender identity or routing.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
- Specific cause
- ipv6 reverse dns missing
- Evidence
- official