All good ideas, thanks. I did check iCal a while back, although I don't tend to use email alarms. Did it more thoroughly just now to make sure, but again no items found. There has been only one item repeating twice a week lately, and the last Mail launch was over a week ago.
Another application doing this would seem to fit the irregularity* of the occurrence, but I have no obvious way of figuring out which one. Since I kill Mail as soon as it pops up, there's nothing there either to give me clues (and yes, I checked Sent items anyway). I also checked Console logs a couple of times, but I couldn't spot any telltale items either.
*) Unfortunately, I haven't kept track of the dates and times of those Mail launches. Suffice it to say that I'm now prepared for future occurrences.
alternaut • moderator