Just received a bounce from Barracuda Reputation used by one of my partners:

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.

xxx@seitelsystems.com Error, ERROR_CODE :554, ERROR_CODE :Service unavailable; Client host [sender1.zohomail.com] blocked using Barracuda Reputation; http://www.barracudanetworks.com/reputation/?r=1&ip=67.183.87.188