IP 208.122.213.26 blocked S3150 — delist tool not sending confirmation emails

Question

IP 208.122.213.26 blocked S3150 — delist tool not sending confirmation emails

Rutger Tolenaar 0

Hi Outlook Postmaster Team,

We are experiencing a persistent S3150 block for our mail server IP

208.122.213.26 since 18 May 2026. We are a small community platform

sending transactional notification emails (low volume).

Steps already taken:

Submitted automatic delist request at https://sender.office.com/ multiple times over the past week
Used two different non-Outlook contact addresses (Gmail and a custom domain hosted elsewhere) — no confirmation email arrives in either inbox or spam folder
Verified our authentication setup is correct: SPF, DKIM, DMARC all pass; mail-tester.com score 10/10
PTR record (msd5855.mjhst.com) has valid forward-confirmed reverse DNS

Bounce message we receive:

550 5.7.1 Unfortunately, messages from [208.122.213.26] weren't sent.

Please contact your Internet service provider since part of their

network is on our block list (S3150).

Could you please:

Investigate why confirmation emails from the delist tool are not reaching our verification addresses
Manually review IP 208.122.213.26 for delisting

Domain: <>

Sender domain SPF: v=spf1 a mx ip4:208.122.213.26 include:spf.brevo.com ~all

DKIM selector: x._domainkey.<>.com

DMARC: v=DMARC1; p=none; sp=none; ...

Thank you for your time.

0 comments

2 answers

Your answer

Answer 1

Hi Rutger,

Your IP is indeed on a blacklist. Not even IP, the complete range.

Please fill this form since you are dealing with consumer accounts.

https://olcsupport.office.com/

Another option will be to email: delist@microsoft.com

In your message, include:

text
Subject: Manual Mitigation Request – S3150 Block – 208.122.213.26 (Since 18 May 2026)

IP Address: 208.122.213.26
PTR/rDNS: msd5855.mjhst.com (forward-confirmed)
Network Owner: MOJOHOST B.V.
Error: 550 5.7.1 ... (S3150)

Steps already completed:
- Multiple sender.office.com delist submissions (confirmation emails not received)
- SPF: Pass | DKIM: Pass | DMARC: Pass
- mail-tester.com score: 10/10
- No listings on MXToolbox (61 blacklists checked), Spamhaus, CBL, Talos
- SNDS enrolled: [add once done]
- Low-volume transactional-only sending (notification emails for community platform)

Request: Manual review and mitigation of S3150 block — automated portal non-functional for our IP.

You can also ask the Hosting Provider if they can provide any input.

I hope this information helps. If you have any more questions, don't hesitate to reach out.

Otherwise, if your problem is resolved, please take a moment to rate the answer. Wishing you good luck!

Answer 2

For an S3150-style block where messages from a specific IP are rejected and the delist portal at https://sender.office.com is not completing (no confirmation email received), the supported actions are:

Follow the instructions in the NDR exactly
- For IP-based blocks, Microsoft requires that external senders use the Anti-Spam IP Delist Portal at https://sender.office.com when the NDR points there.
- If the NDR instead contains a different instruction (for example, to forward the NDR to a specific address such as delist@microsoft.com for 5.7.511 cases), that path must be used.
Verify that the correct delist workflow is being used for the error code
- For generic IP blocks (e.g., 550 5.7.606–649 Access denied, banned sending IP [x.x.x.x]), the documented path is the Office 365 Anti-Spam IP Delist Portal at https://sender.office.com.
- For 550 5.7.511 Access denied, banned sender [x.x.x.x], the delist portal cannot be used; instead, the full NDR must be sent to delist@microsoft.com with the IP address included. Microsoft then responds within about 48 hours with next steps.
If the delist portal cannot be completed because the confirmation email never arrives
- The supported guidance for external senders is:
  - Re-attempt the delist request at https://sender.office.com using a working mailbox that can receive external mail.
  - If problems persist, ask a Microsoft 365 recipient (a customer or partner whose users are affected) to open a support ticket with Microsoft 365 Support on behalf of the owner of the blocked IP. External senders typically cannot open Microsoft 365 support tickets directly, and there may be legal reasons Microsoft Support must work with the IP space owner.
Ensure the sending environment meets Microsoft’s expectations
- Confirm that the sending IP does not advertise itself as a private/nonroutable IP (for example, 10.0.0.0/8, 172.16.0.0/11, 192.168.0.0/16) in its HELO/EHLO or reverse DNS, because Microsoft may reject such connections.
- Ensure reverse DNS (PTR) and forward DNS are correctly configured and consistent.
- Maintain good sender reputation: low complaint rates, proper SPF/DKIM/DMARC, and clean lists, as these factors influence SmartScreen and other filtering.
For Outlook.com/consumer scenarios
- If the issue is specifically with Outlook.com (consumer) rather than Microsoft 365 commercial tenants, the dedicated Outlook.com sender support/delisting form must be used. The documented form for Outlook.com is provided in the Outlook.com sender support article, and the FAQ there should be reviewed before submission.

Manual investigation or delisting of a specific IP cannot be performed from this channel. The only supported paths are the delist portal, the delist@microsoft.com workflow when indicated by the NDR, and Microsoft 365/Outlook.com support tickets initiated by affected recipients.

References:

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IP 208.122.213.26 blocked S3150 — delist tool not sending confirmation emails

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