[Plugin: Contact Form 7] Cannot use Cc anymore – causes errors

I have used Cc in the additional headers in the past with no problem.
But now under wordpress 3.0.x I get the following error when the form is submitted :

Failed to send your message. Please try later or contact administrator by other way.

The cc is located under additional headers as suggested in the Contact Form & Docs/Setting up mail:

Cc: [email protected]

When this line is removed, everything works.