I didn’t run out of space, and this happens even when I select exactly one job to re-run so it doesn’t look like a race condition. There’s clearly more than one issue here, and what I’m seeing here is not related to that linked topic.
(It turns out that I didn’t have a certificate problem after all; the server names were in the alternate server name. But I moved to using the hostname that matches the SN anyway, and it made no difference.)
I’m successfully sending a tremendous number of mails, just these few jobs are stuck. I don’t know, for example, what log entries to go looking for to help diagnose.