Short answer: Microsoft runs its own block list. If Microsoft 365 (business) recipients return 5.7.606-5.7.649 Access denied, banned sending IP, use the delist portal at sender.office.com. If Outlook.com / Hotmail addresses return errors containing S3150 or S3140, submit Microsoft's Outlook.com sender support request.
Which error are you getting?
| Error | Recipients | Fix |
|---|---|---|
550 5.7.606-649 Access denied, banned sending IP [IP] | Microsoft 365 business mailboxes | sender.office.com delist portal |
550 5.7.1 ... part of their network is on our block list (S3150) | Outlook.com, Hotmail, Live | Outlook.com sender support request |
550 5.7.1 ... (S3140) | Outlook.com, Hotmail, Live | Outlook.com sender support request |
550 5.7.515 Access denied, sending domain ... does not meet the required authentication level | Outlook.com, Hotmail, Live (bulk senders) | Not an IP block: publish SPF, DKIM and DMARC for the sending domain |
550 5.7.23 / 5.7.26 | All | Not a block, an authentication problem: SPF, DMARC |
Microsoft 365 delisting (sender.office.com)
- First stop the spam source and check your IP against other blocklists too.
- Go to sender.office.com and enter your email address and the blocked IP.
- Click the link in the verification email and confirm the request with "Delist IP".
- Removal usually takes effect quickly (often within 24 hours).
Outlook.com / Hotmail delisting
- Note the code in the error (S3150, S3140, etc.) and the IP.
- Submit Microsoft's Outlook.com sender support (delist) request, including the IP, your domain and the error text.
- Microsoft usually replies with an automated assessment; sometimes the answer is "not eligible" and further information is requested. Replying with details of what you fixed can help.
- For the long term, join Microsoft's SNDS (Smart Network Data Services) to monitor your IP's reputation at Microsoft, and JMRP (Junk Mail Reporting Program) to see complaints from Outlook.com users.
How to avoid being blocked again
- Set up SPF, DKIM and DMARC completely. Since May 2025 Microsoft requires all three from domains sending more than 5,000 messages a day to Outlook.com addresses: check your records.
- The IP's PTR record should point to your mail server's hostname.
- Reduce sending to addresses that don't exist; remove bouncing addresses from your lists.
- When you start sending from a new IP, increase volume gradually (IP warming).
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Frequently asked questions
I'm not on any other list but Hotmail won't accept my mail. Why?
Microsoft uses its own reputation system, which doesn't map one-to-one to public blocklists. Note the S code in the error and submit Microsoft's sender support request.
Does Microsoft charge for delisting?
No. Both sender.office.com and the Outlook.com sender support request are free.