Often, there will be more Received lines following the first dishonest host. At this point, it's time to send mail. You need to send a note saying what happened, and you must include the spam itself, with its entire header intact. Without that header, the sending system can't track down the individual user who sent the spam, and your complaint will generally just be ignored. If you're running a mail server on the Internet and you're able to send or receive mail, you're required to have a postmaster address, and it's required that you monitor it.
Postmaster is the default contact for any domain.
Having an abuse address is an informal standard that has cropped up among the larger ISPs. If they have an abuse user, they generally request that you send spam complaints to that address, and reserve postmaster for connectivity and delivery problems. Figure 8 shows some sample mail that gets the point across. Note the second paragraph about their system being used to relay spam. If it did, it'll be up to them to trace it.