CCO? What mail server are you using? If they are BCC'ed to the user, without the mail server adding a header to the email saying who the email was delivered to (ie. a Delivered-To header), Gossamer Mail has no way of determining who the email was intended for. qmail adds the Delivered-To header by default, and you can do something similar with sendmail by following the instructions mentioned here:
http://www.sendmail.org/faq/section3.html#3.29
Adrian
http://www.sendmail.org/faq/section3.html#3.29
Adrian