You can send emails to your customers within the Customer Service interface in Store Commander, accessible from the Customers menu.
Once in the interface, simply click on the icon on the toolbar.
The 'send email' window will open, where you enter the name of the customer, and automatically the email address will popup so you can just select it.
And then select the shop, if you are working in Multistore mode, the subject of your email, and your message.
You also have the possibility to add an attachment to your message.