Speaking of spam

[Jonathan Hutchins]

I have one domain that generates a dozen or so bounce messages to the postmaster each day from 
spam, usually addressed TO legitimate addresses in that domain.  I'm not sure exactly what these 
are, maybe they're attempts to bounce the spam back to illegitimate origins.

Thing is, it's only that one domain, out of a half dozen or so on the same server.

Any guesses what's going on here?