Select Utilities---->>>>Send Bill Due Reminder
1.Select the Billing Company
2.Select the Customer
3.Select the bills which needs to be added in the mail and enter the Subject
4.Select attach the invoices in case you want to send the pdfs
5.Click on Send Reminder
The content of the mail is predefined and can be modified.
In this way you can send mail to the client for all his outstanding invoices attached in bulk in a single mail.