You can use the CLAMPMail (previously called Quickmail) feature in Moodle to email all or a selected group of your students:
In the middle of the page, on the right, will be a box containing "Potential groups" and below that a box with "Potential recipients." The "Potential groups" box will list the groups you have created in the course (if you have created any groups). The "Potential recipients" box will show all the students enrolled in the course, along with any teaching assistants. (Please note that it's not problematic if you haven't created groups and if the list of potential recipients says "Not in a group.") If you want to send an email message to a particular group of students, you would click on one of the groups in the “Potential groups” box, and that would highlight the names of the students who are part of that group. You would click on the Add button in the middle of the screen to add all those particular students to the “Selected recipients” list.
To send an email message to all students in the course, click on the button in the middle of the page: