Bookem doesn't do email campaigns, only bulk SMS campaigns (see how). However, you can use other bulk emailing apps, such as MailChimp, by exporting your client list from Bookem and importing them into in any of these apps.
To do this, head to Clients, click on Export Clients on the top right.
Alternatively, download only clients that have had bookings in the past by downloading all appointment data directly from your calendar. You can do this by going to Calendar and clicking on the top right and Download.
Then choose Spreadsheet (CSV), choose your date range and click 'Download'. See below.