>DMARC and traditional mailing lists don't play nice for two reasons: the 
>subject is modified to add a prefix,
>which invalidates the DKIM signature, and the body is modified to add a 
>footer, which again invalidates the DKIM
>signature.

It's more than that.  Lists do all sorts of other stuff, from adding
body headers to reordering, stripping, or flattening attachments.

When we were doing DKIM we spent a lot of time trying to figure out
whether there was a way to make signatures that would survive mailing
list modifications with little success.  The l= option was intended to
allow lists to add footers, but I don't know of anyone who uses it.

To answer your specific question, I don't think modified subjects are
a huge threat, but the subject line isn't the problem.

R's,
John

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