Outlook and Hotmail blocks: the delist portal, SNDS and what each one fixes

Microsoft does not publish a queryable blocklist for Outlook.com, Hotmail, Live or Microsoft 365. What it has is internal reputation data, a set of rejection codes that tell you roughly which system objected, and — for IP-level blocks against Microsoft 365 — a self-service delist portal. Knowing which of those applies to your rejection decides whether there is a form to fill in at all.

Decode the rejection

Copy the exact code out of the bounce before you do anything else. The difference between an IP block with a portal and a sender-level ban without one is the difference between a ten-minute fix and a fortnight of reputation work.

The three Microsoft tools worth having

The Microsoft 365 anti-spam IP delist portal. For S3140/S3150-style IP blocks, Microsoft offers self-service delisting: you submit the IP and an email address and receive confirmation. It is genuinely quick. It is also not a reputation reset — if the cause persists, you will be blocked again, and repeated submissions do not help.

SNDS (Smart Network Data Services). If you own the sending IP, register it for per-IP data Microsoft will not otherwise share: complaint rates, trap hits, traffic volume and a colour-coded status. This is your only real visibility into the Outlook.com side.

JMRP (the Junk Mail Reporting Program). A feedback loop that forwards Outlook users' spam reports for your IPs. Enrol, and suppress every reported address permanently and immediately.

Note what is absent: there is no lookup that tells you whether your *domain* is in bad standing, and no portal that clears a domain-level problem.

Recovery, in order

1. Read the code, and check whether the block is IP-level (portal-eligible) or sender-level (not).
2. Confirm the IP is yours. On Google Workspace, Microsoft 365, SES, SendGrid, Mailgun or Postmark, it is not. Escalate to the provider and work on your domain instead.
3. Find the cause before any submission. Check for compromise first — a hacked mailbox or an abused form is the most common reason for a sudden Microsoft block. Then look at volume shape and complaint sources.
4. Fix authentication for alignment. SPF and DKIM passing, at least one aligned with the From domain, DMARC published. Microsoft enforces this directly, and 5.7.509 says so.
5. Correct reverse DNS on any IP you control, forward-confirmed, with a matching HELO.
6. Enrol in SNDS and JMRP so the next incident is visible before it becomes a block.
7. Submit to the delist portal if the code is IP-level, once the cause is resolved.
8. Resume at low volume to your most engaged Microsoft recipients and rebuild over one to two weeks.

What not to do

• Do not submit the portal form repeatedly. It is self-service and it is logged. Repeat submissions with no change behind them work against you.
• Do not look for a Hotmail blocklist to check. There is nothing to query; SNDS is the closest thing and it only covers IPs you own.
• Do not treat a deferral as a block. A 451 4.7.x is throttling. Keep volume low, retry normally, and let it clear.
• Do not rotate domains. Microsoft weights domain history heavily, and a new domain starts with none.
• Do not ignore 5.7.708. Range-level blocks are about the netblock you are in. If your provider cannot resolve it, that is information about your provider.
• Do not forget Spamhaus. Corporate Microsoft 365 tenants frequently add third-party lists of their own, so a Spamhaus listing can be the real reason a specific tenant refuses you.

The part with no portal

Sender- and domain-level standing at Microsoft recovers through changed behaviour over days to weeks, and nothing in Microsoft's tooling will confirm it. What you can observe is the deferral rate falling and placement in Outlook mailboxes returning, which means the measurement has to be yours.

MailPilot measures per-provider inbox placement daily, including Outlook and Microsoft 365, warms mailboxes back up at a controlled pace after a block, and verifies SPF, DKIM and DMARC alignment — so you can tell a genuine recovery from a quieter continued block.