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Establish a list for students who are enrolled in the class. During class throughout the first several weeks ask students to let you know if they are not on this list. Students have differing opinions on how often emails should be sent, so feel free to ask them their opinions during the midterm survey. A summary email of events once a week usually works well with reminder emails for more important things such as assignment deadlines. Include links to anything you want them to read, such as assignments.
Listserv
The AguaClara listserv can be used for general announcements that are not for class. The list has hundreds of students who have been interested in the project at some point but are not necessarily enrolled in the class. It can be used, for example, for events such as the benefit concert or lectures that might be of interest. You can also send emails to announce the speakers for class since students not enrolled might want to attend.