Send all headers to [email protected]

They will take care of the rest.
If they trace the culprit he'll probably just get a warning but wo'nt get kicked off.
If the person has a record at his/her isp then he will be in deeper ****.
If the ISP is a free ISP then you can forget it.

If the attacker is some kiddie who used Kaboom then they can definately trace him.
If the person is a lot smarter and wrote/used a script to have a anonymous remailer take care of the delivery then things get harder since the ISP then needs a court order to force the anon remailer to cough up all logs.

It's easy to know it there whas an anon remailer involved, did the person send more or less then 1000 mailbombs?
more means no remailer (they can't handle such a load) or if there are more remailers involved (headers can easily tell you that) less then 1000 could be both.