I'm sending the message from my home computer, a G5, OS 10.3.4, using Entourage (both 2001 and 2004).
I'm using my POP account.
Can't remember what the message says. I may have to wait until I send out another email to 70 or so addresses.
I'm not sure where the message comes from, whether it's from Dreamhost, or my ISP (comcast).
I'll try to report back the next time I see the error.