To email a customer, click Contacts in the left menu and navigate to the Customers main feature page.
Once the page opens, locate the customer you want to contact and click the checkbox to select their row. Above the customer list, there will be an option to E-Mail the customer.
A popup will ask which email service you want to use to open the email options. Click the email service you normally use from the list and you will be able to compose and send an email with the customer's email auto-populate.