redirected emails are bouncing.

[phingers]

So I've setup user@domain.com to forward to their ISP email account.

Messages are coming in that are spam, and are being redirected to user@ispdomain.com

I then get a mailer-daemon@server.domain.com email back

XXX.XXX.XXX.XXX does not like recipient.
Remote host said: 554 <spam@spamdomain.com>: Sender address rejected: Domain not found. See http://www.url.com/spam-rejected.html
Giving up on XXX.XXX.XXX.XXX.

Perfect, the ISP rejects the spam, yet I wind up with a message still.

Ideas?

PS. the domain is in plesk setup with reject as the bounce for unknown users.