Forum Moderators: mack
One is to have your mail server expand the addresses. You can do that by setting up a mailing list, or an alias with your mailserver and sending to that. You can use something like mailman from list.org to manage it, allowing your users to subscribe and unsubscribe as they want. You could also keep them from making use of those features. The mail you send out will have the address of the mailing list in the "To: " header. I like this method since it allows people to unsubscribe themselves.
Another way is to have your mail client send an individual email to each user. You can do this with a mailer like you usually use to read your mail, but the more common method is to use a script to iterate through a list of addresses and send an individual mail to each person. The "To: " line on each mail will have the recipient's address in it.
There are other methods, like sticking everyone in the "Bcc: " line, but they're not as common or straightforward, I think.
Good luck, and don't spam!
-Andy