Customer statements can be e-mailed by going to Customers > Selecting the Customer > Click the Invoices Stats Etc tab > Click Statement of Account button.  On the next screen Click the E-Mail Statement button at the bottom of the page.  You'll then be prompted for the customers e-mail address (this automatically fills if you've got one set in the Customer Profile), a subject and message text.  When you're ready just press the Send Email button.