Outlook 451 4.7.650: “temporarily rate limited due to IP reputation” (S775)

Short answer: Outlook.com is slowing down mail from your server's IP because of its reputation. It is a temporary deferral, not a block: messages are retried, and the limit lifts once Microsoft sees clean traffic. Fix what hurts the IP's reputation and, if it lasts for days, ask Microsoft sender support to review it.

The bounce looks like this
451 4.7.650 The mail server [203.0.113.10] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S775)

What it means

Outlook.com, Hotmail and Live accept only part of your mail and defer the rest because the sending IP has a weak or unknown reputation: a new server, a sudden jump in volume, spam complaints or spam from a compromised account. A 4xx code means "try again later", so your server keeps retrying. It turns into a bounce only when the retries run out. A hard block shows up as 550 5.7.606 or 550 5.7.1 S3150 instead.

How to fix it

  1. Check the domain below: SPF, DKIM and DMARC should all pass. Microsoft weighs authentication heavily for new IPs.
  2. Look at your outgoing queue and logs for spam: a hacked mailbox, website form or script sending in bulk is the most common cause of a sudden S775.
  3. Slow down: send less mail to Outlook.com addresses per hour and grow volume gradually; drop old, unengaged and bouncing addresses.
  4. Make sure the server's reverse DNS (PTR) matches its host name and the IP is not on Spamhaus.
  5. Sign up for Microsoft SNDS (IP reputation data) and JMRP (complaint reports) to see what Microsoft sees.
  6. If it doesn't clear within a few days, open a request with Outlook.com sender support and include the full bounce. Owners report that the first reply is often an automatic "no issue detected"; replying to ask for escalation is what got the limit raised.

Check your domain now

After the fix, send a test email to see SPF, DKIM and DMARC pass on a real message, then keep an eye on it: free DMARC monitoring shows every service that sends mail as your domain and which ones fail, in plain English.

Rather have it unblocked for you? I find why Microsoft blocked your server, fix SPF, DKIM, DMARC and reverse DNS, and file the delist request, following up until Microsoft answers.

Get it unblocked on Fiverr

Prefer to do it yourself? Fix Kit ($15) for SPF, DKIM and DMARC, or the VPS Security Kit ($19) so a hacked server can't get the IP blocked again.

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