Fix Microsoft 365 Error 451 4.7.500-699 (ASxxx) by Creating a Connector

Overview

If Microsoft 365 is rejecting messages from our platform with an error such as 451 4.7.500-699 (ASxxx), it usually means Microsoft has temporarily throttled mail from the sending IP because the sending pattern changed or the IP does not yet have an established reputation. Microsoft recommends creating a connector to trust the sending service and remove this throttling. 

More info: https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/fix-error-code-451-4-7-500-699-asxxx-in-exchange-online

Why this happens

Microsoft 365 uses anti-spam protections that can temporarily limit messages from IP addresses that suddenly begin sending at a higher volume than usual. This is part of Microsoft's filtering and IP throttling process. If your inbound mail first passes through a third-party service like ours, Microsoft advises setting up a connector with security restrictions.

How to create a connector in Microsoft 365

To ensure reliable email delivery, create a connector in Microsoft 365 that allows messages from our platform to be trusted and accepted.

Step 1: Open the Exchange Admin Center

1. Go to the Exchange Admin Center: https://admin.exchange.microsoft.com
2. In the left-hand menu, expand Mail flow
3. Select Connectors
4. Click Add a connector

Step 2: Configure the connector type

Set the connector options as follows:

• Connection from: Partner organization
• Connection to: Office 365

Click Next.

Step 3: Name the connector

Enter a name such as:

Trusted Mail Connector

You can also add an optional description.

Make sure Turn it on is enabled, then click Next.

Step 4: Verify by IP address

Choose the option to identify the partner organization:

By verifying that the IP address of the sending server matches one of the following IP addresses

Add these IP addresses:

• 3.106.21.22
• 13.237.47.221

Click Next.

Step 5: Enable TLS protection

Enable this setting:

Reject email messages if they aren't sent over TLS

Click Next.

Step 6: Review and create

1. Review the configuration
2. Click Create connector

What this does

This connector tells Microsoft 365 to trust messages sent from our platform when they come from the approved IP addresses and are delivered securely over TLS. This helps prevent messages from being blocked, throttled, or quarantined unnecessarily. Microsoft specifically recommends configuring a connector when inbound mail is routed through a third-party service and you are seeing this ASxxx throttling error.

After you finish

Allow a few minutes for the connector to become active, then send a test email to confirm delivery is working as expected.

If you still see the error after creating the connector, contact our support team and include the full bounce message so we can help investigate further.