Use Bcc

[Mahmud]

Use Bcc: when forwarding emails and tell your friends to do the same:
  This is So important. When folks pass along an email containing multiple email addresses in the To: line, they are broadcasting their friends' and others' email addresses to strangers as the email makes its rounds. Eventually, a spammer will get the email and all of the email addresses. Be nice to your friends. If you want to forward an email, put your address in the To: box and put your friends' addresses in the Bcc: box. [In Yahoo, gmail after clicking Forward, click "Show bcc" underneath the From line. In Google, click Add Bcc.]