Signed header defaults break mailing lists

[fweimer-rh]

The defaulted signed headers include usually non-existing mailing list related headers such as List-Id:

pdkim/pdkim.c

Lines 44 to 51 in 2ef12ef

	#define PDKIM_DEFAULT_SIGN_HEADERS "From:Sender:Reply-To:Subject:Date:"\
	"Message-ID:To:Cc:MIME-Version:Content-Type:"\
	"Content-Transfer-Encoding:Content-ID:"\
	"Content-Description:Resent-Date:Resent-From:"\
	"Resent-Sender:Resent-To:Resent-Cc:"\
	"Resent-Message-ID:In-Reply-To:References:"\
	"List-Id:List-Help:List-Unsubscribe:"\
	"List-Subscribe:List-Post:List-Owner:List-Archive"

This has the unfortunate effect that the non-existence of these headers is asserted. As most modern mailing list software adds these headers, such a signed message cannot be distributed across a mailing list without breaking or stripping the signature.

(Originally reported against Exim.)