Last night I changed the Postfix configuration to accept mail for hewgill.com. I knew there was some misdirected mail that made it there, but I had no idea how much. Since last evening (perhaps 15 hours ago), I've received nearly 400 junk messages to that server. This is happening even though there is no MX record at all pointing to that server (for hewgill.com)!
All of it is worm messages of some kind. Evidently there is some flavour of worm that looks up an A record for the domain name, instead of an MX record, when trying to deliver mail. Now, once I move my MX record to point to the hosted server, I won't be able to distinguish that worm mail from real mail anymore.
Now I just need to integrate clamav into Postfix and check for all that junk worm mail with virus attachments at SMTP time.