Dear Community, My company is an e-mail service provider undergoing IP address changes as we've switched data center IP transit provider. Microsoft's mail servers are only accepting a few messages a day from our new IP address, while the volume of mail we send towards Microsoft365 domains is in the thousands per hour. The error we receive is 451 4.7.500 Server busy. When we use the old IP address for the same messages, mails get sent successfully. What I checked and tried already: Verified we're not on a blacklist. I used MXToolbox for this Verified the configuration of the postfix server is in order, it's a clone of the old server, but with hostname and DNS entries updated with a valid PTR record. I checked the Smart Network Data Service, this lists all IPs as normal status I put in a support request with OLC Support to check if anything is blocking, the reply I got was "Nothing was detected to prevent your mail from reaching Outlook.com customers.", even though we keep getting the above error message. Are there any other steps I can take? With the volume of e-mail we're sending it's hard to build a reputation if the door is being automatically shut after a few hundred mails at the start of the day.

Hi，@ Rednas Dekkers Thanks for posting your question in the Microsoft Q&A forum. According to your description, you changed the IP address and then the number of emails you can send was limited. I checked the official documentation and found that this error is expected behavior. You'll get this error when the source IP address that's sending you email changes significantly from its previously-established patterns. This part of a filtering technique known as graylisting : when new senders appear, they're treated more suspiciously than senders with a previously-established history of sending email messages (think of it as a probation period). For this type of error, Microsoft officially provided a solution: set up a connector in the mail system. Fix NDR error 451 4.7.500-699 (ASxxx) in Exchange Online - Exchange | Microsoft Learn You can refer to this link for the function of the connector: Configure mail flow using connectors in Exchange Online | Microsoft Learn If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.

A lot Deferred mail with error 451 4.7.500 Server busy when mailing to MS365 domains

Rednas Dekkers 0

Dear Community,

My company is an e-mail service provider undergoing IP address changes as we've switched data center IP transit provider.

Microsoft's mail servers are only accepting a few messages a day from our new IP address, while the volume of mail we send towards Microsoft365 domains is in the thousands per hour.

The error we receive is 451 4.7.500 Server busy.

When we use the old IP address for the same messages, mails get sent successfully.

What I checked and tried already:

Verified we're not on a blacklist. I used MXToolbox for this
Verified the configuration of the postfix server is in order, it's a clone of the old server, but with hostname and DNS entries updated with a valid PTR record.
I checked the Smart Network Data Service, this lists all IPs as normal status
I put in a support request with OLC Support to check if anything is blocking, the reply I got was "Nothing was detected to prevent your mail from reaching Outlook.com customers.", even though we keep getting the above error message.

Are there any other steps I can take?

With the volume of e-mail we're sending it's hard to build a reputation if the door is being automatically shut after a few hundred mails at the start of the day.

Bruce Jing-MSFT 6,025 Reputation points Microsoft Vendor

2024-07-16T06:06:49.9033333+00:00

Hello, @Rednas Dekkers

It's been a long time since I've heard from you, have you solved your problem yet? Did my previous reply help you？ If it was useful, you can mark it as an answer. Thank you for your understanding and support.
Bruce Jing-MSFT 6,025 Reputation points Microsoft Vendor

2024-07-22T05:59:57.3866667+00:00

Hi，@Rednas Dekkers

Following up to see if the below answer was helpful. If this answers your query, do click Accept Answer and Yes for was this answer helpful. And, if you have any further query do let us know.

1 answer

Bruce Jing-MSFT 6,025 Reputation points Microsoft Vendor

2024-07-12T06:10:54.8966667+00:00

Hi，@Rednas Dekkers

Thanks for posting your question in the Microsoft Q&A forum.

According to your description, you changed the IP address and then the number of emails you can send was limited.

I checked the official documentation and found that this error is expected behavior. You'll get this error when the source IP address that's sending you email changes significantly from its previously-established patterns. This part of a filtering technique known as graylisting: when new senders appear, they're treated more suspiciously than senders with a previously-established history of sending email messages (think of it as a probation period).

For this type of error, Microsoft officially provided a solution: set up a connector in the mail system.

Fix NDR error 451 4.7.500-699 (ASxxx) in Exchange Online - Exchange | Microsoft Learn

You can refer to this link for the function of the connector:

Configure mail flow using connectors in Exchange Online | Microsoft Learn

If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.
Please sign in to rate this answer.
Rednas Dekkers 0 Reputation points

2024-07-12T07:30:42.87+00:00

Hi @Bruce Jing-MSFT , thank you for your answer.

Isn't a connector made in MS365 by a MS365 customer only for their domains? If that's the case, it's not an solution as the domains our customers are e-mailing to, are quite dynamic

Bruce Jing-MSFT 6,025 Reputation points Microsoft Vendor

2024-07-12T08:06:31.4933333+00:00

Hi，@Rednas Dekkers

I am glad to hear from you.

Based on the information you provided, since you have a large number of customers with different domain names, creating a connector is indeed not a good approach.

For email security reasons, Microsoft will restrict IP addresses that have undergone major changes. Based on your current situation, this restriction cannot be lifted for the time being, and you need to complete this trial period.

If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.

Rednas Dekkers 0 Reputation points

2024-07-12T08:13:06.5733333+00:00

Hi @Bruce Jing-MSFT

The thing is, last night (Amsterdam time zone) we were able to send ~500 mails/hour just fine. This morning we got rate limited again after just 200 mails in an hour.

Any info on how long this trial period is?

I do understand why it is in place, but I wonder how bigger providers "fix" this, as I can imagine providers sending millions of mails/hour adding IP addresses to their mail cluster. Our new IP went from zero reputation and not sending e-mail (its been idle for a year since we obtained the IP block), to sending ~50K mails/day.

Bruce Jing-MSFT 6,025 Reputation points Microsoft Vendor

2024-07-12T08:36:22.03+00:00

Hi，@Rednas Dekkers

I'm sorry this issue can't be resolved at the moment.

Here are my suggestions:

Ensure You're Following Email Best Practices:

Ensure that your server is not sending spam or malware.

Make sure you have a correct reverse DNS record set up for your mail server.

Ensure your server supports and is correctly configured for SPF, DKIM, and DMARC. Microsoft 365 checks these records to evaluate the legitimacy of emails.

Rate Limiting:

Microsoft's servers may temporarily rate-limit IPs if they detect a large volume of emails in a short period, especially if the content is seen as suspicious. Ensure you're not sending a high volume of similar emails in a short time.

Office 365 Delisting:

If your IP has been blocked or rate-limited by Office 365 (Outlook/Exchange Online Protection), you can request delisting through the Microsoft SNDS (Sender Network Data Services) and JMRP (Junk Mail Reporting Program) services.

Register at the Microsoft SNDS website to get data about your IP's reputation with Microsoft and request delisting if necessary.

Contact Microsoft:

If you've followed all the best practices and are still facing issues, consider reaching out to Microsoft support for assistance.https://learn.microsoft.com/en-us/microsoft-365/admin/support-contact-info?view=o365-worldwide

Consider External SMTP Relay:

Consider using an external SMTP relay service, which might have better delivery rates to Microsoft 365. There are many SMTP relay providers like SendGrid, Mailgun, etc., that offer such services.

Lastly, while the error is "temporary", repeated occurrences indicate that there's an underlying issue that needs to be resolved. It's essential to diagnose and correct the root cause to ensure smooth email delivery in the future.Ensure You're Following .
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A lot Deferred mail with error 451 4.7.500 Server busy when mailing to MS365 domains

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