The Email Statements field (available only with Keystroke Advanced licenses) works with the Accounts Receivable - Print Forms For - Statement Settings when emailing Statements for Customers.
If Email Smts is enabled, the Customer's statement is emailed when the Print Form For Statement settings "Email Customers Only", "Both Print and Email Customers", and "All Customers" are selected.
If Email Smts is disabled, the Customer's statement is emailed when the Print Forms For Statement settings "No Print/Email Customers Only" and "All Customers" are selected.
See:
Customer Database - Main Screen - Print Statements
Customer Database - Main Screen Fields