Question : Host or domain name not found - Name service error

When I email one of my clinets I get the following error, any thoughts on this:

The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.

< smtp.crossan.ie #5.0.0 X-Postfix; Host or domain name not found. Name service error for name=DublinCityuBID.ie type=A: Host not found>

Answer : Host or domain name not found - Name service error

"< smtp.crossan.ie #5.0.0 X-Postfix; Host or domain name not found. Name service error for name=DublinCityuBID.ie type=A: Host not found>"

That says it all. It looks like you're trying to email [email protected], but that domain doesn't exist. It's not even registered.