On Tue, 1 Nov 2016, Jim Cheetham wrote:
Hi Mailop,
We run our listening mail servers with a maximum header size limit of 32768
(Sendmail's default).
We've found at least one "legitimate" sender whose headers are far bigger than
that,
and the reason for this isn't a very long path :-) it's all the
"x-microsoft-exchange-diagnostics-untrusted" multiline headers being
dumped in there by one of the O365 tenancies outside our control.
Obviously we're asking the sender to see if they really need to have
all that header data added, but in the meantime I'd like to ask this
community whether you see that 32KB max header size as a sensible limit
in today's conditions?
Exim has a configurable option "header_maxsize" which defaults to 1MB
which I have never adjusted.
The default was 1MB as far back as June 2010 - as far as I can trivially
check.
(Exim also has a second configurable option
"header_line_maxsize" which
limits the length of any individual header line in a message,
after all the continuations have been joined together.
Messages with individual header lines that are longer than the limit are
rejected.
The default value of zero means no limit.
).
I have also recently seen many messages with long multi-line
headers with detailed spam-scoring diagnostics and the like,
but so far I've not seen any that caused problems.
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