Well, I did finally get to the message header. By my reading:

• It was received from the sender by "mcsv3.net (PowerMTA(TM)" at Wed, 10 Feb 2010 08:21:24 -0500

• It was received by Yahoo! Wed, 10 Feb 2010 11:03:42 -0800, more than 5 hours later.

There's no mention of anything else handling it in between. It sounds as if it arrived in my Yahoo! inbox right after I posted the preceding.