Ah, OK. Looking at the MailChimp page, it appears that these headers appear on
a message being sent to MC, and then it extracts them, most likely removes them
from the final generated email, and uses them as processing instructions on how
to generate the email or sequence of emails. In any case it seems rather
unlikely that they should ever appear in a received email message.
And whether they should or not, the values given for about 90% of the headers
is simply invalid according to the MC page describing them. The headers that
are valid are direct copies of the examples given on the MC page, and would not
likely work for any real email campaign.
I'd say that presence of X-MC-xxx headers in a received message is a 100%
guarantee of a targeted advertizing message, and a 99% guarantee that the
message is a spam. If the values given for the options in the headers are
obviously invalid, that rises to a 100% chance that the message is spam.
I'd call these headers a great spam sign.
Loren
