If you are sending your mail by visiting
http://mail.yahoo.com via a web browser, then mail can't be being redirected to your network admin.
However, your admin might be checking all mail type data baing sent using a packet sniffer or similar. If this is the case sign up for a gmail account as this is done over a secure web connection (HTTPS) which yahoo MIGHT not be.
If you are using an e-mail client such as Outlook express or Thunderbird, then your admin might be using a smarthost or mail relay to redirect your mail going out through port 25 - double check the BCC field as well to make sure it's not already filled in though. One way round this is to check if yahoo offers secure e-mail on an alternative port; either SSL or TLS should work fine.
Rgds
Havelock