550 4.4.7 QUEUE.Expired: message expired
Exchange Online kept retrying for 24 hours but couldn't deliver the message, so it expired in the queue and bounced.
#550 4.4.7 QUEUE.Expired; message expired ##
The diagnostic line, as shown in Microsoft's article on 550 4.4.7.
What 550 4.4.7 means
RFC 3463 defines X.4.7 as "Delivery time expired": the message was considered too old by the rejecting system, because it stayed on that host too long or its time-to-live passed. RFC 3463 adds that, where possible, the code for the actual problem should be returned instead.
Microsoft calls 550 4.4.7 a persistent transient failure. The destination server was unable or unwilling to accept the message and kept returning temporary 4.x.x errors. Exchange Online retries for 24 hours, and only then sends this NDR. Microsoft's NDR reference adds that it can also mean a header limit was reached on a remote server or another protocol timeout happened.
If it happens only for one domain, the destination is the likely problem: its server is offline or unreachable, won't accept the message, has missing or wrong MX records, or has anti-spam settings blocking you. Microsoft also lists causes on the sender's side: an incomplete SPF record, an expired domain, or a hybrid configuration problem. Microsoft also lists "AmbiguousRecipientTransientException", where Exchange Online found several directory objects matching the same recipient.
Likely causes
- The destination mail server was offline, unreachable or kept deferring.
- The destination domain's MX record is missing or wrong.
- The destination's anti-spam settings block mail from you.
- Your SPF record is incomplete, or your domain has expired.
- Duplicate recipient objects (AmbiguousRecipientTransientException) or hybrid misconfiguration.
How to fix it
If you sent the message
- Check whether the problem affects one domain or many.
- Tell the recipient by another channel, and include the NDR with the destination server's last error.
- Run the Outbound SMTP Email test in Microsoft's Remote Connectivity Analyzer.
- Check your SPF record and that your domain registration is active.
If you're the recipient or their admin
- Check your MX records and that your mail server accepts connections from Exchange Online.
- Review anti-spam settings that may block Microsoft 365 senders.
Frequently asked questions
How long does Exchange Online retry before 4.4.7?
Microsoft says Exchange Online retries delivery over 24 hours. Only after 24 hours of failed attempts does the sender get the 4.4.7 NDR.
Why does a 4.x.x code arrive in a bounce?
The 4 shows the underlying failure was temporary: the destination kept deferring. The message bounced because the retry window ran out, not because the destination refused it outright.
Sources
- Fix NDR error "550 4.4.7" in Exchange Online (Microsoft Learn)
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
- RFC 3463: Enhanced Mail System Status Codes
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
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