Unrouteable address

I modified a website contact form for an organization so it could be used for a mailcampaign. The intention was that they would write a text, paste it into the form, and all their members would have to do was to sign it (name, firstname and emailaddress) and press send, and it would be send to several politicians. I did warn them that this might come back and bite them in the ass. A couple of dozen should be enough to get the politicians attention (maybe up to a 100), but at certain point, more will just piss them off. And it went wrong somewhere between the 300th and 400th mail. They are very disappointed because they had hoped for up to 10,000 mail. (apparently they prefer quantity over quality).

There is nothing wrong with the code, before their campaign and after they got into trouble I tested it out on my linode, and everything worked fine. Though it never crossed my mind to let them use my linode for their campaign. They have my sympathy, but I doubt the effectiveness of their methods.

This is the error message they get when sending the email:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  serverroot@example1.nl
    (ultimately generated from info@example2.be)
    Unrouteable address
  hb@example3.be
    Unrouteable address
  qwerty@yahoo.com
    Unrouteable address

3 different email addresses on 3 different mailservers became unrouteable. Is this because they were blocked on the receiving end (simultanously)? Or were they blocked by their hosting provider?

I told them they should consider this campaign as over, but I am interested as to what caused the error message.