Microsoft documentation states that they support message headers up to 256KB
https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#message-limits:~:text=Message%20header%20size%20limit Regards, Graeme -----Original Message----- From: mailop <mailop-boun...@mailop.org> On Behalf Of LANEL, JEAN BAPTISTE via mailop Sent: Wednesday, August 28, 2024 9:46 PM To: A. Schulze <s...@andreasschulze.de>; mailop@mailop.org Subject: Re: [mailop] maximum header size Hello, Yes I already noticed that MS is adding plenty of headers but 64 kB seems a bit high, I check on an email I retrieved from my FBL the headers are something like 15 kB (more than half of it being from headers starting with X-MS or X-Microsoft). So 64 kB seems rather high (I think it is over the commonly admitted limit of 50 kB) maybe the messages are looping amongst various relays before reaching your MTA ? JB -----Message d'origine----- De : mailop <mailop-boun...@mailop.org> De la part de A. Schulze via mailop Envoyé : Tuesday, August 27, 2024 7:48 PM À : mailop@mailop.org Objet : [mailop] maximum header size Caution! External email. Do not open attachments or click links, unless this email comes from a known sender and you know the content is safe. Hello, today a became aware of messages deferred by our MX. I could nail down the issue to "all headers excced 64k bytes". This limit was enforced by OpenDKIM [1] Deferring is questionable. I would prefer reject but that's an other story. The messages in question were sent by an MS365 tenant. - Do you see such 'huge' message headers, too? - Do you handle/allow larger headerdata? - How much do others accept? To be clear: it's not the size of one header but all message headers submitted so far. Andreas [1] https://manpages.debian.org/bookworm/opendkim/opendkim.conf.5.en.html#MaximumHeaders _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop Worldline, Cardlink, GoPay and Santeos are registered trademarks and trade names owned by the Worldline Group. This e-mail and any documents attached are confidential and intended solely for the addressee. It may also be privileged. If you are not the intended recipient of this e-mail, you are not authorized to copy, disclose, use or retain it. Please notify the sender immediately and delete this e-mail (including any attachments) from your systems. As e-mails may be intercepted, amended or lost, they are not secure. Therefore, Worldline’s and its subsidiaries’ liability cannot be triggered for the message content. Although the Worldline Group endeavors to maintain a virus-free network, we do not warrant that this e-mail is virus-free and do not accept liability for any damages, losses or consequences resulting from any transmitted virus if any. The risks are deemed to be accepted by anyone who communicates with Worldline or its subsidiaries by e-mail. Please consider the environment before printing, sending or forwarding this email. _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop Email secured by Trustwave advanced threat protection. Learn more at https://trus.tw/mailmarshal This transmission may contain information that is privileged, confidential, and/or exempt from disclosure under applicable law. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution, or use of the information contained herein (including any reliance thereon) is STRICTLY PROHIBITED. If you received this transmission in error, please immediately contact the sender and destroy the material in its entirety, whether in electronic or hard copy format. _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop
