Deferred mail with error 451 4.7.500 Server busy when sending mail to domains hosted Microsoft office 365

Question

Deferred mail with error 451 4.7.500 Server busy when sending mail to domains hosted Microsoft office 365

habieb zain 25

Dear Community.

My company's mail server has an issue when sending emails to domains hosted on Microsoft 365.
I use postfix as the smtp mail server engine.
In the queue log, log like this:

288A2628411A 656263 Wed Oct 11 14:13:56 ******@ourdomain.web.id
(host RECIPIENT-DOMAIN1-com.mail.protection.outlook.com[XXX.XXX.51.138] said: 451 4.7.500 Server busy. Please try again later from [202.XXX.XXX.XXX]. (S77714) [DBAEUR03FT057.eop-EUR03.prod. protection.outlook.com 2023-10-11T09:27:36.542Z 08DBC9BA9FB2C3B1] (in reply to end of DATA command))
                                          ******@RECIPIENT-DOMAIN.com

This is permanent error? 
Is there any suggestion so that I can send emails smoothly immediately?

Regart;
Abeb

Ali AlEnezi 1,116 Reputation points

2023-10-11T11:56:14+00:00
Hi Abeb,

The error message you're seeing (451 4.7.500 Server busy. Please try again later) is a temporary one, but it can be quite persistent if Microsoft's servers have identified any suspicious patterns or issues with emails coming from your server.

Here's what you can do to address the situation:

Ensure You're Following Email Best Practices:

Ensure that your server is not sending spam or malware.

Check your server's reputation. Sites like mxtoolbox.com and senderbase.org can help you see if your server IP is on any blacklists.

Make sure you have a correct reverse DNS (PTR) 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.

Investigate Further:

Examine the postfix logs for any other warning or error messages.

Check to see if this is happening to all emails to Microsoft 365 domains or only some. This can help determine if it's a widespread issue or related to specific emails or recipients.

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.

Consider External SMTP Relay:

As a workaround, if you're facing persistent issues with sending directly from your Postfix server to Microsoft 365 domains, 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.
Ajay Rooplall 15 Reputation points

2023-10-11T12:04:16.6533333+00:00

Hi,

I am having the exact same issue, log error the same. This only started today. Never had this issue.
habieb zain 25 Reputation points

2023-10-12T04:04:58.5+00:00

Hi Ali AlEnezi,

Thanks so much, very deep explanation.

Our email queue is clear. Maybe because it's been almost 8 hours since yesterday.

but at least I got a hint on how to solve this problem.

Hi Ajay Rooplall

I've been using Postfix for almost 2 years, and just yesterday I had a bad experience sending to domains hosted by Microsoft.
Linh Phan 0 Reputation points

2023-11-07T07:49:56.85+00:00

It happens again !!!
S.F. Garcia - AFB Communications BV 0 Reputation points

2024-02-19T13:44:18.4233333+00:00

i have the same issue since a couple of days before that never had problems
Amol G 0 Reputation points

2024-02-26T18:20:50.07+00:00

Getting same issue on with IP 176.9.84.87 too SMTP error from remote mail server after end of data: 451 4.7.500 Server busy. Please try again later from [176.9.84.87]. (S77719) [AMS0EPF000001A2.eurprd05.prod.outlook.com 2024-02-26T18:02:44.983Z 08DC2EE1B7BE86CB]

How do we solve this?
Lluis Grau Amenos 0 Reputation points

2024-10-18T09:34:13.1066667+00:00

I have had this problem for 5 days now. It appears on different domains and from different countries and IPs. Does anyone know how to solve it?
GP 0 Reputation points

2025-05-07T16:04:43.2+00:00

Hello,

We have the same issue described in this post. We investigated our environment but didn't find any problem. We suspect our IP is rate limited / greylisted by Microsoft 365 filters.

Is there any way to contact MS support who can help with delisting our IP?

(E-mail / Form /Phone contact, which actually works)

Answer accepted by question author

Evgeniy Goldshtein 76

Exactly the same problem. Sometimes queue on our mail relay grows huge for an hour, then suddenly all emails go out and then start to be delayed again

habieb zain 25 Reputation points

2023-10-12T04:07:17.7166667+00:00

Hi Evgeniy Goldshtein.

I don't know what happened to Microsoft. after almost 7-8 hours, my email queue was cleared.

3 additional answers

Your answer

Ajay Rooplall 15 Reputation points

2023-10-11T12:04:16.6533333+00:00

Hi,

I am having the exact same issue, log error the same. This only started today. Never had this issue.
habieb zain 25 Reputation points

2023-10-12T04:04:58.5+00:00

Hi Ali AlEnezi,

Thanks so much, very deep explanation.

Our email queue is clear. Maybe because it's been almost 8 hours since yesterday.

but at least I got a hint on how to solve this problem.

Hi Ajay Rooplall

I've been using Postfix for almost 2 years, and just yesterday I had a bad experience sending to domains hosted by Microsoft.
Linh Phan 0 Reputation points

2023-11-07T07:49:56.85+00:00

It happens again !!!
S.F. Garcia - AFB Communications BV 0 Reputation points

2024-02-19T13:44:18.4233333+00:00

i have the same issue since a couple of days before that never had problems
Amol G 0 Reputation points

2024-02-26T18:20:50.07+00:00

Getting same issue on with IP 176.9.84.87 too SMTP error from remote mail server after end of data: 451 4.7.500 Server busy. Please try again later from [176.9.84.87]. (S77719) [AMS0EPF000001A2.eurprd05.prod.outlook.com 2024-02-26T18:02:44.983Z 08DC2EE1B7BE86CB]

How do we solve this?
Lluis Grau Amenos 0 Reputation points

2024-10-18T09:34:13.1066667+00:00

I have had this problem for 5 days now. It appears on different domains and from different countries and IPs. Does anyone know how to solve it?
GP 0 Reputation points

2025-05-07T16:04:43.2+00:00

Hello,

We have the same issue described in this post. We investigated our environment but didn't find any problem. We suspect our IP is rate limited / greylisted by Microsoft 365 filters.

Is there any way to contact MS support who can help with delisting our IP?

(E-mail / Form /Phone contact, which actually works)
habieb zain 25 Reputation points

2023-10-12T04:07:17.7166667+00:00

Hi Evgeniy Goldshtein.

I don't know what happened to Microsoft. after almost 7-8 hours, my email queue was cleared.

Answer 1

Khaled11 0

Hi All

We have new published record, and we are getting the same form some domains:

below site listed our IP as blacklisted

UCEPROTECTL3

After searching, we figured out that the whole subnet is listed which belongs to GCP

We have many emails being Queued because of this reason

0 comments

Answer 2

Mary 0

I had the same problem till I started using turbo-SMTP.com as my SMTP server.

0 comments

Answer 3

Serge IT 0

Solved.

Thanks.

habieb zain 25 Reputation points

2023-10-12T04:08:29.31+00:00

Hi Serge,

i will try add that IP.