Success! I’ve been busy and hadn’t gotten around to troubleshooting this again. lamp’s message reminded me. I followed the steps (also running applejack when I restarted) and was able to get the mail plug-in installed.
I spend more time in DTPO than almost any other app on my Mac and this is the only thing that ever bothers me about it. Why would this be a problem for some and not others, I wonder. Oh well…
/christopher