Comment 4 for bug 558208

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I think this is problematic. For intra-organizational lists, removing the
DomainKeys header works as advertised. However, if a person from a
DomainKeys-enabled domain posts onto an external list, there is a potential
for error.

If the sender's domain's DomainKeys settings specify that the domain does
not send unsigned mail, external MTAs can and will drop an email from that
domain if the DomainKeys headers are removed.

To make DomainKeys work with mailman as expected, admins have two
possibilities:
1) Deliver the message as-is, without modifying the Subject header or
message body (or any header indicated to be signed). The original
DomainKeys signature will then work.
2) If modifying the message is necessary, the mailing list will have to rewrite
the From: header in order not to claim that the message originated in the
DomainKeys -protected sender domain.

Removing the DomainKeys header will only be relevant in the case 2) above.
For 1) -- the preferred solution for many lists -- it is actively harmful.
Therefore, automatically removing the DomainKeys header is NOT the way to
go.