451 4.7.650: Temporarily Rate Limited Due to IP Reputation (Microsoft)

A 451 4.7.650 means Outlook.com (Hotmail, Live, MSN and Outlook.com addresses) is temporarily refusing to take more mail from your sending IP because it doesn't trust that IP's reputation right now. It's a deferral, not a block: your server is expected to back off and retry, and mail that's retried patiently usually gets through. If the deferrals keep going for days, though, Microsoft is telling you something about your volume, your list or your complaints.

What the bounce says

A real example, posted to Microsoft Q&A, with the identifying details replaced:

451 4.7.650 The mail server [203.0.113.7] has been temporarily rate
limited due to IP reputation. For e-mail delivery information, see
https://aka.ms/postmaster (S775) [Name=Protocol Filter Agent][AGT=PFA]

The S775 and agent details may differ in your logs. Some senders also see 4.7.651 with the same wording.

What it means

451 is the SMTP reply for "requested action aborted, local error in processing," and the 4 at the start of 4.7.650 marks it as a temporary failure (per RFC 3463, the 7 means a security or policy reason). In plain terms: "not now, try later."

Microsoft hasn't published a definition of 4.7.650. Its Outlook.com postmaster pages document a related family of 421 RP-001 to RP-003 codes (rate, connection and per-connection limits) and say the reason for those limits "is related to IP/domain reputation." A Microsoft moderator on Q&A described 4.7.650 the same way: something about the IP's sending behavior, such as volume, recipient engagement or content, looked suspicious.

This applies to Outlook.com consumer mailboxes. If you're bouncing at Microsoft 365 business domains, see the 550 5.7.606 to 5.7.649 page or the 550 5.7.708 page instead.

Common causes, most likely first

1. A new or cold IP. Microsoft's postmaster site says IPs not previously used for email "typically don't have any reputation built up." Sending a full campaign from a new dedicated IP is the classic trigger.
2. A volume spike. A big send after a quiet period, or a sudden jump in daily volume, looks the same as a cold IP.
3. Complaints. Microsoft calls the junk complaint rate "one of the principal factors in driving down a sender's reputation." Mail to people who don't remember signing up gets reported.
4. List quality. Lots of unknown users and old addresses (which can include spam traps) push reputation down.
5. Shared IP neighbors. On an ESP's shared pool, someone else's bad send can rate-limit you. See my ESP's shared IP is blocklisted.
6. Missing authentication. Outlook.com requires SPF, DKIM and DMARC from domains sending more than 5,000 messages a day. Failures there usually produce 550 5.7.515, but weak authentication doesn't help reputation either.

How to fix it

1. Let retries work. Don't panic-resend or move the queue to another IP. A 4xx deferral means your MTA should keep the message and retry with increasing gaps. Check that it does, and that it isn't treating 4.7.650 as a hard bounce.
2. Reduce pressure on Outlook.com. Lower concurrent connections and messages per hour to Microsoft consumer domains until the deferrals stop. Most ESPs and MTAs let you throttle per destination domain; if you're on an ESP, ask them to.
3. Send to your most engaged subscribers first. Recent openers and clickers generate positive signals and few complaints. Hold older segments back until the IP is accepted normally.
4. Register for SNDS and JMRP. Smart Network Data Services gives you Microsoft's view of your IPs, and the Junk Mail Reporting Program sends you reports when people mark your mail as junk. Both are on the Outlook.com postmaster site. Use JMRP to remove complainers.
5. Check your authentication. Run your domain through the email checker to confirm SPF, DKIM and DMARC pass and align.
6. Fix the list problem behind it. Suppress hard bounces, stop mailing long-inactive addresses, and look for any recent import or new sign-up source.
7. If it persists after the fixes, open a ticket. Use the Outlook.com sender support form (it requires a Microsoft account sign-in). Include the IP, the full bounce text, and what you've changed. The Microsoft delisting guide walks through it.

How to confirm it's fixed

Watch your MTA or ESP logs for Microsoft consumer domains. You're done when retried messages are being accepted, the queue for Outlook.com drains, and new sends go through on the first attempt. Then check placement, because accepted isn't the same as inboxed; the Outlook junk folder guide covers that side. Look at the trend over several days, not a single send.

How to keep it from coming back

• Warm new IPs gradually, starting with engaged recipients. Microsoft says a new IP on an authenticated domain can expect to be fully ramped "within a couple of weeks or sooner."
• Keep daily volume to Outlook.com steady instead of saving everything for one big send.
• Keep your JMRP and SNDS registrations updated whenever IPs change; Microsoft specifically reminds senders to add new IPs to JMRP.
• Sunset unengaged subscribers so complaints and unknown users stay low.

If 4.7.650 turns into a hard rejection like S3150, go to the S3150 guide. For an overview of Microsoft blocks, see why Outlook is blocking your emails.

When to get help

If deferrals last more than a week after you've slowed down and cleaned up, or Microsoft support says your IP is fine while the deferrals continue, it's worth getting a second set of eyes on your data. Get in touch.

FAQ

Is 451 4.7.650 a bounce?

Not a permanent one. It's a temporary deferral, so your server should retry. It only becomes a bounce if retries run out before Outlook.com accepts the message, usually after several days depending on your MTA's settings.

How long does 4.7.650 last?

Microsoft doesn't say. Deferrals from warming a new IP usually ease as consistent, well-received mail builds reputation; deferrals caused by complaints or list problems last until you fix those.

Does a 4.7.650 affect Gmail or Yahoo?

No, it's Microsoft's code and only affects Outlook.com consumer mailboxes. The same underlying reputation problem can show up elsewhere, though, as Gmail's 421 4.7.0 or Yahoo's TS codes.

Can I use sender.office.com to fix 4.7.650?

No. The sender.office.com delist portal is for Microsoft 365 business blocks. Outlook.com consumer issues go through the Outlook.com sender support form.