Mass mailing from Mail

Yes. Go to:

Window > Address Book

Then click on one of the contacts, press Command + A to select all the contacts and then click the 'To:' button.

And you're done!

Note that if you follow the solution adrian-polglase suggested, all of the addresses will be blasted out in the "To:" header of the message, so every recipient will see the email address of every other recipient, and any of them can retain the addresses or hit "reply all" and send to all of them. If that is what you intended, you can ignore this comment.

If you want something more private, use the "Bcc:" button.

If you want a separate or customized message to each recipient (without crafting them yourself), you'll need to use another program.

Additionally, there are limits on the number of addresses that can be used at one time, set by both application and/or the email provider.