Mail delivery Failure 554 5.6.211

Question

Hi

Has anyone ever seen such an error message from Exchange Online:
Mail Delivery Failed Mail delivery failed via SMTP server 'None'. SMTPDataError: 554 5.6.211 Invalid MIME Content: Number of Header objects (100001) exceeded allowed maximum (100000). [GV0P278MB0115.CHEP278.PROD.OUTLOOK.COM 2023-09-27T07:55:23.823Z 08DBBDF2E27CA54A] [AM4PR07CA0023.eurprd07.prod.outlook.com 2023-09-27T07:55:23.827Z 08DBBEC4AD87E831] [AM7EUR03FT029.eop-EUR03.prod.protection.outlook.com 2023-09-27T07:55:23.826Z 08DBBE03F9D3F08A]

A customer has a system that sends a lot of emails through EXO.
Sometimes they have this error message in the log of their local smtp system, which is not an Exchange Server.
I was searching for this error, but didn't find a usefull answer.
I also checked using message tracking, but when the error appears, there were never so much emails sent.

Any idea what that error exactly means?

Regards
Peter

Answer 1

Hello Peter

Based on your description, it looks like this may be caused by interference from other email systems. I think you need to start with the local smtp system.
<<Sometimes they have this error message in the log of their local smtp system, which is not an Exchange Server.

the log comes from the local smtp system or both?

Just want to check, are you in Europe?

Regards

Shaofan