Problems using CC: notifications using tkgnats

[Raquel Toribio]

I am trying to use the mail notification feature of tkgnats.
Whenenver I am in the menu View Problem Report and I select the Send Email
button, and fill the CC: field with an e-mail address (ex. emailxxx), I get
the following error in tkgnats:
SMTP error sending RCPT TO:
<emailxxx>
550 5.7.1 <emailxxx> ... Relaying denied.

And in the server I get in the /var/log/maillog:

 LAB sendmail[28991]: f5FCISD28991: ruleset=check_rcpt, arg1=<emailxxx>,
 relay=pc01 [192.168.0.101], reject=550 5.7.1 <emailxxx>... Relaying denied

Note: pc01 is my PC, where i have installed tkgnats. And LAB is the PC where
i have installed gnats.

Do I have to set something in order to make the relay through pc01 work? How
does the mail notification facility work?