why my emails come back when sent through CCI

[sophie]

i need to send a mailing to 300 people. I sent 49 contacts through the CCI so
that the recipient can't see the other contacts, but all the emails have come
back to me. I have a message saying the host or domain was not found although
if i send the email one by one, it will go through. I use Outlook and my
server is neuf telecom.

Any idea why??

Thanks for your help

sophie