E-Mail sending to Offce/Microsoft365 customers delays - 451 4.7.500 Server busy

Question

We have changed the IP of our mail server due to switching hosting. Now our Mail server can not send emails to outlook/office 365 users.

451 4.7.500 Server busy. Please try again later from [OUR IP]. (S77719) [RECIPIENTADDRESS.prod.outlook.com 2023-11-01T03:18:16.434Z

This problem has been persistent for several days. We do not experience any problems with email delivery to any other services. Hotmail, Gmail and numerous other private services do not delay or reject our emails at all. On the other hand, we have thousands of messages to our clients that are being constantly delayed. We do not have this delay occurring with ANY other service except the recipients that use *protection.outlook.com. We saw that this issue has occurred to multitude of people (issue reference EX680695) in early October and was at that time resolved. Perhaps this issue is related because we are showing exactly the same symptoms?

We have been talking to outlooksupport@microsoft.com and they claim that there is nothing that should be preventing our emails from reaching Microsoft customers. At the time I am writing this I can no longer communicate with them because now our emails addressed to outlooksupport@microsoft.com are also indefinitely delayed.

We triple checked all the settings and everything is correct, and we ONLY experience the problem when trying to send to *prod.outlook.com addresses. Does anyone have any suggestions what to do?

Answer 1

Hi @Damir Akhoundov ，

Welcome our Q&A forum!

According to your description and NDR report, the problem seems to be that your recipient cannot receiving the email or email delayed, and there is nothing to do with you as the sender side.

It is recommended that you contact the recipient and ask them to submit service help case and seek help from MS, and I have searched in our IM and have not found similar cases so far, I will let you know if there is.

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