Meanwhile, the site that's actually rejecting your mail has made that decision *itself*,
that it doesn't want to receive mail from you, possibly with MAPS as one component
of the information used to make said decision.
To have a chance of winning this argument, you'll have to prove that the receiving
system is legally *obligated* to accept every piece of mail that you might happen to
want to send.
MX <> recipient. If I send email foobar@xxxxxxx to the published MX for aol.com and aol.com blocks the ultimate recipient from receiving that email, they are in violation of the law, having interfered with the availability of email for both the sending and receiving systems. By publishing an MX, they have agreed to accept email for any valid address within their domain. That's what an MX is. They are likely in breach of their civil contract with the recipient, also.
So, by that argument, an MX site for FooISP (which presumably would be run by FooISP, or at least under contract with FooISP) can't use Spamassassin or Greylisting to avoid spam either, nor any other criteria. The spammers must love you.
An ISP using such tools may be in breach of contract with the recipient; it depends on the contract, of course. More likely, I suspect, the contract gives them the latitude to prevent abusive behavior, or rejects any guarantees of availability of services at any given time or when dealing with non-local resources. Which is probably enough to cover Spamassassin and RBL.
If you think there's some violation of law going on here, please be more specific. What law, and in what country? As Valdis said, you need to show why they're obligated to accept absolutely every message, regardless of contracts.
Of course, anyone who publishes an MX record but refuses mail is simply an idiot incapable of understanding why the Internet exists in the first place. The Balkanization has begun. The Internet is dead.
Um, right. Maybe my troll filter is on the blink today?
Ken