On Booking Manager portal you can mail your clients directly from your address book. Click on My Clients (1.) and select a contact (2.) you want to send an e-mail to. You may want to check if the contact has an e-mail address entered so click on display client (3.), check for E-mail (4.) and press Send (5.).
You will open a new e-mail addressed to the contact (1.) with a standard subject (2.). Enter the message (3.) and Send (4.).