The original message confused me a little. I could (mis?)interpret it to say that mail that has been given to them, by some outside party, to be relayed to another outside party - since spammers typically attack a mail server outside their (registered via credit card) home provider, who might just whack them with an arbitrary "high message traffic" charge.
In this case, so long as AT&T made the policy clear up front (perhaps by having sendmail reference it in it's greeting) I think they would be in the clear.
I've been tempted to put a "$1000 per non-local origin/destination" charge message on my sendmail banner, and then have my legal department whack Krazy Kevin with a seven-digit default judgement next time he tries a spam run. Let's see ya get a mortgage with that one on your TRW, pal.
Manager, Network Operations 713-307-6760
Wiltel Communications Systems Houston, TX