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As discussed "in Vienna" (see the minutes)
At present there is no limit in the spec on the number of Message-
Instance or DKIM2-Signature header fields that can be present in a
message ....
The only reason that there would be a lot of them is either because
someone is trying to waste your time or because a message is going round
in a loop.
Detecting loops by counting Received: header fields is standard
practice... and there is no need to have more Message-Instance or
DKIM2-Signature fields than Received: header fields in your message
RFC5321 suggests a suitable maximum count of 100 for Received: header
fields but that seems insanely high to me -- out of the box Exim sets a
value of 30 and Postfix 50 -- which again seems high, but there are
large mail systems that seem to pass email through a half dozen machines
so if your mail visits them a few times as it is forwarded then maybe
these values make sense.
Anyway -- I think we should limit the number of M-I and D-S header
fields; and I would suggest 31 as a suitable limit (using a power of two
minus one to allow for fancy coding tricks if you like that sort of
thing).
The change to the spec would be minimal, just documenting the limits and
providing a suitable PERMFAIL message.
- --
richard @ highwayman . com "Nothing seems the same
Still you never see the change from day to day
And no-one notices the customs slip away"
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