Microsoft 451 4.7.500-699 "Access Denied, Try Again Later": Throttling on a New IP

451 4.7.500 to 4.7.699, shown with an ASxxx tag, is Microsoft 365 temporarily throttling mail from your IP. Microsoft describes it as a form of greylisting: the IP's volume rose sharply compared with its history, so mail is deferred while it builds a record. It is temporary and usually clears in a few days.

The 451 4.7.500 code page has the summary.

An example of the text

451 4.7.500 Server busy. Please try again later from [203.0.113.5]. (AS843)

The address in brackets is the sending IP, and the AS tag identifies the rule that fired.

Reading the code

Part	Value	Meaning
Reply code	451	Requested action aborted, local error
Class	4	Persistent transient failure
Subject	7	Security or policy status
Detail	500 to 699	Microsoft's range for these deferrals

Class and subject are defined in RFC 3463. The 500 to 699 detail range is Microsoft's own, which is why the text and the AS tag matter more than the number.

Why it happens

• A sudden increase in volume from an IP Microsoft has little history for.
• A new sending IP or server sending at full volume from day one.
• Mail sent from the tenant's onmicrosoft.com domain.

What to do

1. Let your server keep retrying. Deferred mail is normally accepted as the IP builds history. SMTP deferral vs bounce explains why a 4xx should never be treated as final.
2. Ramp volume up gradually on new IPs instead of starting at full volume.
3. Send from your own domain, not onmicrosoft.com.
4. Check that SPF, DKIM and DMARC pass, so reputation builds on an authenticated identity. See SPF, DKIM and DMARC explained.
5. Do not suppress the addresses. The deferral says nothing about them.

How it differs from a block

A 451 is a deferral, while the 550 codes are rejections. Compare Microsoft 451 4.7.650, which is about IP reputation, and 550 5.7.606, a ban. Outlook bounce codes lists the family.

Where verification helps

Throttling is about volume and history, so verification does not remove it. It does mean the volume you do send goes to real mailboxes, which helps a new IP build a good record faster, because mailbox providers weigh how often a sender hits addresses that do not exist.

Keep the address side clean

This code does not come from your list, but every bounce still counts against your sending reputation, so the addresses you do send to should be real. Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason such as "Mailbox not found" or "Accepts all addresses", and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.