network is on block list. S3140

For a 550 5.7.1 "network is on block list"/S3140 situation when sending to Outlook.com or Microsoft 365 recipients, the path to resolution depends on whether this is Outlook.com (consumer) or Microsoft 365 (commercial) mail.

Because the tag indicates Outlook.com/consumer mail, use the Outlook.com sender support and delisting channels rather than Microsoft 365 enterprise support.

1. Verify the exact NDR text
   • Check the full non‑delivery report (bounce) for:
     • Any reference to a blocked/banned IP or sender.
     • Instructions such as forwarding the NDR to [email protected] or using a delist portal.
   • If the NDR shows a 5.7.606–649 or 5.7.511 style error with instructions, follow those exactly.
2. If the NDR instructs to email [email protected]
   • Forward the full NDR (unaltered) to [email protected] from an admin/operations mailbox.
   • Include:
     • The sending IP address(es).
     • A short description of the mail system and traffic type (transactional, newsletters, etc.).
   • Microsoft will respond within about 48 hours with next steps or a decision on delisting.
3. If the NDR points to the Office 365 Anti‑Spam IP Delist Portal (sender.office.com)
   • Follow the URL in the NDR to the delist portal.
   • Complete the form with the blocked IP, a working email address, and the verification code sent to that address.
   • Submit the request and monitor for the confirmation email and any follow‑up.
4. If you are sending to Outlook.com (Hotmail, Live, MSN) specifically
   • Use the Outlook.com sender support tooling to understand and improve reputation (spam complaints, volume, authentication, etc.).
   • Ensure the following are correctly configured for the sending domain and IP:
     • Reverse DNS (PTR) resolves to a routable hostname and not a private IP range.
     • SPF, DKIM, and DMARC are correctly set up and aligned.
     • The server is not advertising a non‑routable IP (for example, 10.x.x.x, 192.168.x.x, 172.16–31.x.x) in HELO/EHLO.
5. If the NDR indicates the IP is on a third‑party RBL
   • Use the RBL name (if shown) to check the IP on that list and follow that provider’s delisting process.
   • Many environments that use Microsoft services also subscribe to external RBLs; removal must be requested from that RBL operator.
6. Who to speak to / how to reach a human
   • For Outlook.com consumer service, use the Outlook.com sender support and delisting form referenced in the documentation. This is the supported path for IP reputation and block issues; phone/chat agents cannot override blocklist decisions.
   • For Microsoft 365 recipients, external senders must use the delist portal or [email protected] as described in the NDR; there is no direct phone escalation path for non‑customers.
7. While waiting for delisting
   • Reduce or pause bulk traffic from the affected IP.
   • Review logs for possible abuse or compromised accounts.
   • Ensure the server is not an open relay and that outbound spam is not occurring.

If the first mitigation response said the IP was mitigated and a later one said it could not be mitigated, that typically means reputation or policy data indicates ongoing or unresolved issues. In that case, reply to the last delist/mitigation email with:

• Evidence of fixes made (spam stopped, compromised accounts closed, authentication configured).
• A request for re‑evaluation of the IP.

References:

• External senders - Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors
• External senders - Troubleshoot email sent to Microsoft 365
• Sender Support in Outlook.com