Thanks everyone for your input. I do believe this is an issue with the emails from O365 just having a lot of extraneous headers, as I found in my investigation and supported by Soji. I’d rather address this locally by just adjusting the
header size limit, as we have a lot of lists and subscribers using these platforms that don’t restrict their header size. That is better than telling all of our subscribers that their email provider needs to change something, or that we have to strip headers
for them.
The Postfix documentation states in the header_checks man page, “Message headers that are longer than $header_size_limit characters are truncated.” So, why is this even a problem? The default is 102400 and we have not changed it. Yet the
error in the logs (e.g., “stat=Headers too large (32768 max)”) is the sendmail limit [2]. So, I am a big confused.
[1] postconf -d | grep header
always_add_missing_headers = no
canonical_classes = envelope_sender, envelope_recipient, header_sender, header_recipient
detect_8bit_encoding_header = yes
header_address_token_limit = 10240
header_checks =
header_size_limit = 102400
…
[2] grep -ri maxheaders /etc/mail/
/etc/mail/sendmail.cf:#O MaxHeadersLength=32768
/etc/mail/submit.cf:#O MaxHeadersLength=32768
From:
address@concealed <address@concealed> on behalf of IKEDA Soji <address@concealed>
Date: Monday, October 10, 2022 at 10:48 PM
To: Silvers, Tim <address@concealed>
Cc: en@sympa <address@concealed>
Subject: [External] Re: [en@sympa] Headers too large
This message was sent from a non-IU address. Please exercise caution when clicking links or opening attachments from external sources.
-------
Hi Tim,
On 2022/10/11 0:51, Silvers, Tim wrote:
> Hi. We are running Sympa 6.2.68 on CentOS 7 with Postfix as our MTA. We
> had a user report that some emails to their list are not being
> distributed. Logs show “headers too large (32768 max)” errors for these
> messages. The only reference I see to this limit is in Sendmail [1], but
> as I stated, we are using Postfix. I found nothing in the Sympa config
> in reference to header size either, just message size.
>
> Searching this error turns up a lot of references to O365 email headers
> being rather long and running into this default header size limit; the
> sending site is using O365. The fix seems to be for other MTAs to
> increase the header size limit. But how do I do that?
>
> Has anyone else encountered this problem?
>
> [1] /etc/mail/sendmail.cf:#O MaxHeadersLength=32768
>
> /etc/mail/submit.cf:#O MaxHeadersLength=32768
Also on this list en@sympa, there are subscribers that do sometimes not
receive messages due to this problem. (However, on this list, the
problem has occurred only with the site setting MaxHeadersLength to
16384 bytes: See also below).
This problem is essentially due to the fact that some senders does not
care about large header and some receivers have a small header size
limit.
- Such senders include Microsoft Office 365 (Exchange Online) and
certain versions of Cisco IronPort (Email Security Appliance).
- Such receivers include Sendmail and Symantec Email Security.cloud.
An essential solution would be for the senders not to add header fields
that are only useful for their own architecture (*) so that the
recipients with small size of limit (**) won't reject messages.
(*) On o365, it appears to be some sort of bug that the messages are
sent out with some long header fields kept unremoved. There is an
instruction on how to create a transport rules to remove these
fields:
-
https://nam12.safelinks.protection.outlook.com/?url="https%3A%2F%2Fanswers.microsoft.com%2Fen-us%2Fmsoffice%2Fforum%2Fall%2Fe%2F80a3b4a1-d0da-4808-9f04-0942463ddfb3%23LastReply&data=05%7C01%7Ctsilver%40iu.edu%7C9029bc138e92416a6b1308daab33116b%7C1113be34aed14d00ab4bcdd02510be91%7C0%7C0%7C638010533081292391%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=UjGFO2vgELI8X%2Bwz5zzHBZgtfRtxQliuO53ZpnDF0bI%3D&reserved=0
(**) The header size limits in many mail systems are larger than 32768
bytes:
-
https://nam12.safelinks.protection.outlook.com/?url="https%3A%2F%2Fwww.mail-archive.com%2Fmailop%40mailop.org%2Fmsg02856.html&data=05%7C01%7Ctsilver%40iu.edu%7C9029bc138e92416a6b1308daab33116b%7C1113be34aed14d00ab4bcdd02510be91%7C0%7C0%7C638010533081292391%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=5R6auccy7E43iDQWId8hPkD7FNKzlHDxe2Xp%2FYvyRNg%3D&reserved=0
Therefore, administrators of Sendmail are suggested at least not to
set a limit smaller than this, i.e. its default.
Regards,
-- Soji
> Thanks,
>
> Tim
>
