How do I sent an account statement to several customers at once?

Click on the 'Send as e-mail' button under Customer/Reports/Account Statement.

Here you have the option of sending to 'All' customers shown on the screen or to the 'Marked lines' that you are on. (Note: you can select several lines with 'Ctrl+Shift').

E-mail set ups are created under Customer/Maintenance/E-mail set up. Read more here.