What 451 4.7.650 means
Microsoft documents codes in the 4.7.500–4.7.699 range as temporary restrictions: suspicious activity was detected and sending was limited while it's evaluated. 451 4.7.650 is the one senders see most when mailing Outlook.com, Hotmail, Live and MSN addresses, with text saying the mail server has been temporarily rate limited due to IP reputation.
It's a throttle on the sending IP, not a verdict on the recipient. Microsoft accepts some mail and defers the rest; pushing harder keeps the IP in the penalty box.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
451 4.7.650 The mail server [IP] has been temporarily rate limited due to IP reputation.Often followed by an internal reference such as (S775).
Why cold emailers see 451 4.7.650
- A new or cold IP sending meaningful volume to Microsoft consumer domains before it has history.
- A shared IP (your ESP or SMTP provider) whose other customers have hurt its reputation.
- Spam complaints from Outlook.com users, or a high share of mail to invalid addresses.
- Sharp day-over-day volume increases.
How to fix 451 4.7.650
- Let deferred messages retry on a slow backoff. Don't re-queue them aggressively.
- Cut volume to Microsoft consumer domains from the affected IP, then ramp gradually over weeks.
- Clean your list before sending: invalid addresses are one of the fastest ways to lose reputation at Microsoft.
- Register the IP with Microsoft SNDS to see complaint and trap data, and contact your SMTP provider if the IP is shared.
Will it retry on its own?
Don't suppress the recipient. Retry slowly. If it escalates to a 5.7.606–649 'banned sending IP' error, request delisting through Microsoft's sender portal.
Reading the code
The basic code 451 means "Local error in processing, action aborted (temporary)" under RFC 5321. In 4.7.650, the 4 is the class (persistent transient failure) and the 7 is the subject (security or policy). The detail number 650 isn't in the RFC 3463 / IANA registry; it's specific to the provider, so the text after the code carries the meaning.
- Class 4
- Persistent transient failure
- A temporary condition delayed delivery. The message itself is valid, and sending again later may succeed.
- Subject 7
- Security or policy
- A security or policy decision: spam filtering, authentication, blocklists, permissions.
- Detail 4.7.650
- Provider-specific
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.