Comcast undeliverable email errors

Question

How do I resolve undeliverable emails from Comcast emails sent from distribution lists with error code:

Remote server returned '550 5.4.300 Message expired -> 451 4.2.0 Throttled - https://postmaster.comcast.net/smtp-error-codes.php#RL000010'

Answer 1

Hi @Connie Olberg, 

Thank you for reaching out and for including the full bounce message. 

Based on the information you provided, the message is being rejected by Comcast since their server first returns 451 4.2.0 Throttled (code RL000010), a temporary rate limit telling the sender to slow down. Exchange Online keeps retrying, but because Comcast continues to throttle, the message eventually times out and produces the final 550 5.4.300 Message expired. So the underlying reason is that Comcast is rate limiting the mail to their recipients, not anything wrong inside your mailbox. This shows up with distribution lists since a list expansion sends several messages toward Comcast in a short burst from the same shared outbound IP pool, which is exactly what RL000010 is built to slow down. 

To resolve the issue, please reach out to your organization's IT admin to confirm SPF, DKIM, and DMARC are correctly configured and aligned on your sending domain, since Comcast weighs authentication heavily when deciding how much to throttle your IP. 

For detailed guidance, your IT admin may refer to these documentations: 

• How to use DKIM for email in your custom domain - Microsoft Defender for Office 365 | Microsoft Lea… 
• Set up SPF to identify valid email sources for your Microsoft 365 domain - Microsoft Defender for O… 
• Set up DMARC to validate email in Microsoft 365 - Microsoft Defender for Office 365 | Microsoft Lea… 

I hope this information helps clarify your concern. Please feel free to reach out if you have any further questions or need additional assistance.  

Thank you for your time and understanding. Wishing you a great rest of your day.  

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Answer 2

Comcast is throttling messages from the sending environment, which causes Exchange Online to keep retrying until the message expires and generates NDR 550 5.4.300 Message expired. The 451 4.2.0 “Throttled” response is from Comcast and indicates a rate/volume limit on their side, not a local configuration error.

To address this from the sender side:

1. Reduce sending rate and volume to Comcast
   • Lower the number of messages and recipients per minute/hour to @comcast.net addresses from the distribution lists.
   • If possible, stagger or batch large DL sends so they are spread out over time instead of sent in a single burst.
2. Reduce recipients per message
   • Break large distribution list sends into smaller groups so each SMTP transaction has fewer Comcast recipients. This aligns with guidance for 4.4.7/expiration-type issues where too many recipients can contribute to delivery problems.
3. Retry after a delay
   • Because the remote server is temporarily throttling, allow Exchange to retry or resend the message later instead of repeatedly sending immediately. This helps avoid hitting the same throttle condition.
4. Verify recipient addresses and list hygiene
   • Ensure all Comcast addresses in the DL are valid and active. Invalid or stale addresses can contribute to negative reputation and throttling.
5. Check for general delivery issues and NDR patterns
   • Review NDRs and message traces for affected messages to confirm that only Comcast is returning 451 4.2.0 and that other domains are accepting mail normally.
6. Work with Comcast postmaster/support
   • Since the throttle is enforced by Comcast, long‑term resolution typically requires working with Comcast’s postmaster or abuse team (as indicated by their error code documentation) to review sending patterns, reputation, and any rate limits applied to the sending IPs.

From the Exchange Online perspective, 550 5.4.300 Message expired indicates that the message could not be delivered in time because the destination server (Comcast) did not accept it before expiration. The fix is to address the throttling condition on the receiving side (Comcast) and adjust sending behavior so messages are accepted before they expire.

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References:

• Email nondelivery reports and SMTP errors in Exchange Online
• Sent email in Outlook.com comes back "delivery failed"