To send a massive email to your customers we will go to:
Customers -> Customers
Once you see the list of your customers you can press the green arrow button next to the magnifying glass to see more searching options. When the list of the people is exactly the one you want, click on the button "Send mail":
Then you'll see the next screen where you can create the email you want to send to your customer list. Write the subject, attach any files you want and write the email. Once you're don, press "Send" to send the email.
When sending a massive email to your customers make sure the list of customers is the one you want.
Remember to use the blue arrow to select only the ones that should receive the email.