In a nutshell, newsgroups are a means of public discussion.
Mail is a means of private communication.
You determine exactly who is supposed to receive your message, whether it be one person or a group of people. When you send an email 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 of hundreds of thousands of copies of your article, one on each news server that carries the newsgroup, scattered all over the globe.