When I went to look for MailChimp emails, I found that I didn't have any. Apparently, MailChimp's servers are actually blacklisted on my server. And apparently it was done by hand by me about 4 years ago (yay for revision control on config files--boo for terse log comments).
Something major happened about 4 years ago with MailChimp for me to actually construct a permanent blacklist entry by hand rather than let the keywords filter, but I guess I don't remember properly what the issue was anymore.
I checked my own Bayesian and MailChimp does keyword as a medium spam indicator, but not enough for an immediate dump.