I guess I should follow up…  turns out my spam spike was pretty much an illusion.  I wasn’t really getting much more spam than before.  Rather, the mail client on my iMac, which is also the machine I use to automatically check the size of mailboxes, etc, had an issue with one of the indexes it uses (I think).  Each hour it would get confused and redownload a bunch of messages from the server that in fact it already had.  It would do this over and over again, basically as fast as it could.  After rebuilding the mailbox, suddenly all that fake mail was gone, and things were basically back to normal.  (Minus a bunch of spam I had deleted in the mean time.)  You can see the pattern above, showing when the problem started, and when it was fixed.

Not quite back to the original rate, still significantly greater actually, but just as suddenly as the onslaught began, it has subsided.  It would be nice if it returned to where it was before, but I’ll take what I can get for now.