Howdy all:
Had a massive [several hundred processes] spike of exim usage just now. I’ve restarted exim and will monitor things; also, I have a support ticket in with the NOC to see if they can help triage this.
Update, 1228 CST: Well, 1.6 GB of email was sitting in the default address’s account. It’s … now gone. 
That was causing a large part of the problem, because we were getting absolutely mailbombed [dozens of messages a second to non-existent addresses on rmfo-blogs.com]. Now the server rejects any incoming email to rmfo-blogs.com that’s not to a specific address out of hand. [And since there are no mailboxes on the server, that effectively settles the problem.]
I’ll do an audit of all the other accounts on the box and make sure that their default addresses are similarly configured to prevent future issues of a similar nature.