Even if you can figure out the domain the mail came from, complaining to the postmaster there may be a waste of time. The machine the mail came from may actually be run by the people who did the junk mailing. Then you have to figure out who their network provider is and complain to them. Even then, some network providers don't really care or are too overworked to do anything about it. We don't want to be entirely discouraging.
Some service providers do respond well, and pull the account the junk mail is coming from.
The main thing is to bear in mind that your mileage may vary. Whatever the circumstances, this is an abuse of the network and will be treated as such. This document has been plagiarised from an original by Scott Larnach of the University of Edinburgh, to whom my thanks are due.