Thre are two options depending on whether you send them each exactly the same email or a "customized" email (say with their name inserted in the email message...) to each one.
In the send mail dialog, there's a radio button titled "Multiple emails". Use that option and you can email every person in your found set. You can specify a field from your database for the email address in either the To or BCC boxes.
You can also set up a looping script that sends out one email at a time to each recipient in your found set with a calculated body that merges text you specify with data in fields in your database to produce slightly different emails for each recipient.
Please note that not all ISP's are happy to see their clients send out automated mass emails. You may need to work with them to avoid built in limits stopping you from sending out more than a certain number of emails at a time.