andrewf - upon further reflection, since my initial post here seems a little terse, this was originally posted in another (older) thread which discussed the SMTP quota and the possibility that that was causing the error for another user. As such, it was clear that I had already considered and discarded that as a cause of my problem.
I reposted it as a new thread at someone else's suggestion and I probably wasn't fully awake yet, so I didn't recognize the lack of context
I've got a support ticket open. I'll follow up here with the outcome