There are three ways for an instructor to email a class:
Use my.mtholyoke: This is quick and easy and you can send email to the entire class via the Class Roster within For Instructors on the Self-Service menu. It doesn't allow attachments or provide an archive of previous messages sent. See the Registrar's instructions for class rosters(PDF - sign in required).
Course Announcements/News Forum: Go to your course and click on the "Course Announcements" in the first box. Click on "Add a new topic" to write an announcement for your students, and be sure to check "Send forum post notifications with no editing-time delay" option so that it emails the class promptly. Note that this method will not also send an email to you when you email the class, but it does allow attachments, and it will archive posts in the News Forum activity so that you can see the history of announcements you have sent. More on forums.
Use Quickmail: The Quickmail block is found on the top in the right column. Click on "Compose new email" to write a message. Check "receive a copy" to get a copy in your own inbox. Quickmail does not provide as nice of an archive of messages as News Forum does, but it does send you a copy to yourself and it will let you include attachments.