What the bounce says
Exact wording as it appears in the rejection or NDR. Placeholders such as x.x.x.x and example.com stand for your own address and domain.
550 5.4.300 Message expired 550 5.4.300 Message expired, cannot connect to remote server
Message expired is the end state of a long series of temporary failures. Exchange Online tried repeatedly, the destination kept deferring or was unreachable, and the queue lifetime ran out. The interesting information is the last error recorded before expiry.
Most common causes
- The destination server was down for an extended period
- The destination kept returning temporary errors
- A routing or connector problem prevented delivery for the whole retry window
- Greylisting or throttling at the destination never cleared
What to verify next
- Read the last recorded error in the NDR to see why each retry failed
- Verify the destination’s current availability
- Check connectors and routing if the destination is internal or hybrid
- Resend once the underlying condition is resolved
Read the last error
The NDR usually includes the most recent response from the destination: a 4xx deferral text, a connection timeout, or a TLS problem. That text explains why every retry failed and points at the fix. Without it, "expired" only tells you delivery never succeeded.
Common last errors: greylisting that never accepted the retry, a destination throttling the sender for reputation, a certificate problem on a TLS-mandatory route, or a server that was down for the whole window.
- The final deferral reason is the diagnosis.
- Expiry means every retry failed, not that one attempt failed.
- Resending repeats the same sequence unless the cause changed.
Temporary problems that do not clear
Some 4xx conditions are effectively permanent from the sender’s perspective: a recipient throttling your IP for reputation will keep deferring until behaviour changes, and greylisting that keys on a rotating sending IP never sees the retry it expects. Both look temporary and both expire messages.
If the last error mentions reputation or rate, work on the sending side. If it mentions greylisting, use a stable sending IP or ask for whitelisting.
Hybrid and internal routes
Inside a hybrid deployment, expiry frequently means a connector points at an on-premises server that stopped responding, or a certificate on that connector expired. Check the connector’s target and certificate, then resend one test.
Best diagnostic path
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisEmail Infrastructure Digital Twin
Map the domain’s public mail infrastructure and provider relationships before remediation.
Open analysis →Known limits
- The NDR may not always include the final deferral text.
- Retry windows are Microsoft’s and not adjustable by the sender.
Common questions
Was any copy delivered?
No. Expiry means the message was never accepted by the destination.
Should I just resend?
Only after reading the last error and confirming the condition changed; otherwise the resend expires the same way.
Difference from 5.4.316?
5.4.316 specifies connection refused as the reason. 5.4.300 is the general expiry with whatever the last error was.
Why the exact message matters
The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.
Provider reference
For the provider-defined meaning and current requirements, review Microsoft Exchange Online NDR reference.