Is this warning message an Outlook problem?

Question

I have one client that keeps getting this message when he tries to email, he has never had issues before. All my other clients emails are arriving correctly. See error message below. I have only just moved over to M365 in the last week.

Warning: message 1qMxmB-000Dtu-Mv delayed 24 hours

A message that you sent has not yet been delivered to one or more of its

recipients after more than 24 hours on the queue on mx260.antispamcloud.com.

The message identifier is:     1qMxmB-000Dtu-Mv

The subject of the message is: FW: Project update

The date of the message is:    Sat, 22 Jul 2023 09:35:00 +1200

The address to which the message has not yet been delivered is:

  ******@xxxxxxx.co.nz

    host xxxxxxx-co-nz.mail.protection.outlook.com [104.47.71.138]

    Delay reason: SMTP error from remote mail server after pipelined sending

data block:

    451 4.7.500 Server busy. Please try again later from [209.126.121.70].

(S77719) [SY4AUS01FT008.eop-AUS01.prod.protection.outlook.com

2023-07-23T00:50:00.446Z 08DB8A86034A4C25]

No action is required on your part. Delivery attempts will continue for some

time, and this warning may be repeated at intervals if the message remains

undelivered. Eventually the mail delivery software will give up, and when

that happens, the message will be returned to you.

Answer 1

Dear Brad,

Good day! Thank you for posting to Microsoft Community. We are happy to assist you.

Based on your description, you moved over to M365 in the last week, and one of your clients could not send email to your organization and got NDR 451 4.7.500 Server busy. Please try again later.

From the NDR message, it's not Outlook client-side issue. There is possibility that the sender's IP is throttled by M365 mail server, in this situation, would you please firstly try to add the sender IP into your IP allow list: Configure the default connection filter policy | Microsoft Learn. (It takes some period to take effect.)

If the issue persists, we may need to involve the higher-level support team to check whether the sender is block by Microsoft. In this situation, we suggest you go to Office365 admin center>Support>New Service Request.  The support engineers there have the correct escalation channel, and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket: Ways to contact support for business products - Admin help. The engineers in related team have higher permission and resources than us to help check the root cause from background directly.

I hope the information can be useful for you. If there is any update or I misunderstand you, please feel free to let me know. I will continue to assist you. Your understanding and co-operation are highly appreciated.

Thanks for your precious time. Have a nice day！

Sincerely,

Sherry | Microsoft Community Moderator