Mail is private communication.
When you send an e-mail message, the final result is one copy of the message in each specified recipient's private mailbox. News is public communication. You have no control over who will receive your message, except by your choice of newsgroup. When you post a news article, the final result is tens or hundreds of thousands of copies of your article, one on each news server that carries the newsgroup, scattered all over the globe. Some allow you to do both simultaneously, that is, post a public response and e-mail a copy to the original author.