The mailing list for listmasters using Sympa

[IKEDA Soji <address@concealed>]

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://answers.microsoft.com/en-us/msoffice/forum/all/e/80a3b4a1-d0da-4808-9f04-0942463ddfb3#LastReply

(**) The header size limits in many mail systems are larger than 32768
    bytes:

      - https://address@concealed/msg02856.html

    Therefore, administrators of Sendmail are suggested at least not to
    set a limit smaller than this, i.e. its default.


Regards,
-- Soji

> Thanks,
>
> Tim