Problem with prod.outlook said: 451 4.7.500 Server busy. Please try again later

Question

Hi

Some clients send us mail with confirmation of receipt of the message (via server) And such messages are limited by you which prevents everyone from working. The limits appeared two weeks ago. No spam is sent or anything.

Z8Qt64332z1Q9w6 12122 Fri Mar 7 13:42:38 MAILER-DAEMON

(host hopagmbh-de01b.mail.protection.outlook.com[52.101.170.1] said: 451 4.7.500 Server busy. Please try again later from [xxx.xxx.xxx.199]. (S77719) [FR1PEPF00000F10.DEUP281.PROD.OUTLOOK.COM 2025-03-07T13:23:27.902Z 08DD5AA1E03E0308] (in reply to end of DATA command))

                                           ******@xxxxxxxx.de

4Z8Q843pdqz1Q9y3 11902 Fri Mar 7 13:09:40 MAILER-DAEMON

(host imperf-com-pl.mail.protection.outlook.com[52.101.73.8] said: 451 4.7.500 Server busy. Please try again later from [xxx.xxxx.xxxx.199]. (S77714) [AM4PEPF00027A6B.eurprd04.prod.outlook.com 2025-03-07T13:23:28.462Z 08DD5BCA3753DCF4] (in reply to end of DATA command))

                                           ******@xxxxxxxx.pl

4Z8Rhc3zMjz1QB7m 12278 Fri Mar 7 14:19:28 MAILER-DAEMON

(host cliphotel-pl01b.mail.protection.outlook.com[52.101.68.36] said: 451 4.7.500 Server busy. Please try again later from [xxx.xxx.xxxx.199]. (S77719) [DB5PEPF00014B91.eurprd02.prod.outlook.com 2025-03-07T13:23:28.706Z 08DD5CE0AB35455C] (in reply to end of DATA command))

                                           ******@xxxxx.pl

4Z8MZX3sBmz1QB0Q 12240 Fri Mar 7 11:13:56 MAILER-DAEMON

(host senetic-com.mail.protection.outlook.com[52.101.68.5] said: 451 4.7.500 Server busy. Please try again later from [xxx.xxx.xxx.199]. (S77719) [DB1PEPF00039231.eurprd03.prod.outlook.com 2025-03-07T13:23:28.622Z 08DD5D502D66706D] (in reply to end of DATA command))

                                           ******@xxxxx.com

and more

Answer 1

Hi @natan jamal,

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

Based on your description, you are experiencing a 451 4.7.500 Server Busy error. You are receiving this NDR because the source email server (the connected IP address) has changed its previous email delivery pattern by sending a much larger number of messages than in the past.

This error code is part of the anti-spam screening in Microsoft 365 or Office 365. You will receive this error when the source IP address sending you email has changed significantly from a previously established pattern. This part of the filtering technique is called grey-listing: when new senders show up, they are treated more suspiciously than senders with a previous history of sending emails (think of them as being on probation).

This document describes a fix for the NDR error code ‘451 4.7.500-699 (ASxxx)’ in Exchange Online, which you can refer to.

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

If the answer is helpful, please click on “Accept answer” as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.

Thank you for your support and understanding.