network is on block list. S3140

Question

network is on block list. S3140

Brian Duncan 20

I run a mail server and you are blocking my emails, you said 550 5.7.1 network is on block list. S3140. who can I speak to to sort this out

First subsmission said that the server IP was mitigated, then a follow up a week late said the IP could not be mitigated.

I need to speak to someone about this

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2 answers

Your answer

Answer 1

Brian Duncan 20

I have tried, they said they cannot help to get in touch with outlooksupport@

I ma sendi8ng very little email out, the IP is not on a block list.

mxtoolbox.com gives a clean bill of health. I have 3 other IP's which I've mitigated over last few years.

EmilyS726 272.3K Reputation points Independent Advisor

2026-05-21T14:37:28.4233333+00:00

Did you use the Delist Portal to submit your inquiry? https://sender.office.com/
Brian Duncan 20 Reputation points

2026-05-21T15:03:34.2833333+00:00

yes, more than once.

thanks

Brian Duncan 20

here is what I got from that avenue.

Thank you for contacting Microsoft Online Services Technical Support.  It appears that this is a deliverability issue with Outlook.com.  


Outlook.com has a separate support process.

EmilyS726 272.3K Reputation points Independent Advisor

2026-05-21T15:20:15.6166667+00:00

How do you run your server, in your own place via ISP, or via third party company?
Brian Duncan 20 Reputation points

2026-05-21T15:25:27.65+00:00

own dedicated server. have 3 others. over last few years I have had to get the IP's mitigated.

so I am assuming Microsft needs to do the same at their end, this time.
Brian Duncan 20 Reputation points

2026-05-21T15:29:09.97+00:00

its a new server, 1 month, very little traffic so their is no spamming.
EmilyS726 272.3K Reputation points Independent Advisor

2026-05-21T15:44:37.34+00:00

I am surprised that they told you it was an outlook delivery issue, because it clearly wasn't.

You did also say that they wouldn't migitate it this time? If Microsoft already replied that the IP “could not be mitigated,” that unfortunately is not something a forum volunteer, independent, or normal support agent can override. Public blacklist checks such as MXToolbox can be clean while Outlook.com still blocks based on its own internal reputation data.

Have you used the SNDS portal to request data for your IP so far? https://sendersupport.olc.protection.outlook.com/snds/Index
Brian Duncan 20 Reputation points

2026-05-22T08:24:21.24+00:00

thanks for your feedback, appreciated. yes used SNDS 3 times, firsty time it said would be mitigated in 24-48 hours, after 7 days I resubmitted and said it is not able to be mitigated, and repeated again a few days ago.

yes I was kinda hoping someone from microsoft would help out, as I'm having dificulty getting clarification and it fixed. I believe they have to do something and I was getting nowhere.
EmilyS726 272.3K Reputation points Independent Advisor

2026-05-22T15:26:19.03+00:00

That is the only support channel for this type of issue. Regular support won't have access to override their decision.

If Microsoft won't delist your IP, I am afraid you might have to ask your ISP to move you to a different one.
Brian Duncan 20 Reputation points

2026-05-22T16:00:12.3066667+00:00

in fact, I got a response this morning, as I resubmitted, asking for the error messages and more details, they are going to esscalate it.

so perhaps they will action it now. thanks very much for helping.
EmilyS726 272.3K Reputation points Independent Advisor

2026-05-22T16:01:30.55+00:00

Oh good. I am glad they responded. That's really the only channel that would help you.
Brian Duncan 20 Reputation points

2026-05-24T17:00:17.61+00:00

got anothe rresponse saying it is now mitigated, so hoepfully OK now. just need to keep contacting them to resolve.
EmilyS726 272.3K Reputation points Independent Advisor

2026-05-24T17:10:44.5666667+00:00

I am glad you stayed on top of that. I hope it is ok now too.

Answer 2

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.

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.
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.
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.
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.
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.
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.
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: