Gmail recognizes the "Booking request" as spam \ Email impostor its not good
i think, its will be Better if the sender's e-mail will email Booking request
Then it will be possible to answer the customer directly from the email to approve or reject the Booking request
NOTE: I want to donate this plugin
if you give me a link to make a donation
http://wordpress.org/extend/plugins/simple-booking-form-wordpress-plugin/