About half of the messages in this thread ended up in my spam folder (I use
gmail as my mailbox with a custom domain).
From looking at the message headers, I noticed what all the messages
flagged as spam seem to have in common: dkim signatures. The dkim
signatures are all failing because the body of the message is being altered
with the footer message that mailman appends. That was obviously also
happening with the old list, but it's reputation likely prevented that
specific issue from being considered as highly as it is with this new
listserv.
I also noticed that neither listserv domain has spf records published. If
a proper spf record can be published, and new messages start passing spf
checks, perhaps the dkim failures won't weigh as heavily anymore???
I think the only way to prevent the dkim signatures from failing in the
first place is to either replace the email address in the "From: " header,
or to prevent modification of the message body by disabling the footer
insertion.