Thanks to people on the Japanese mailing lists, I’ve learned that
iso-2022-jp should be used for mail and news because it uses 7 bits
instead of 8. For some reason, my Emacs defaulted to euc-jp. Fixed
now with the handy snippet

(setq mm-coding-system-priorities
       '(iso-2022-jp iso-2022-jp-2 shift_jis iso-8859-1 utf-8))

Don’t worry about M-x describe-coding-system not saying that the
buffer will be in iso-2022-jp. If you have

Coding system for saving this buffer:
  = -- emacs-mule

it will try iso-2022-jp first.