You need 2-step verification on your '''googlemail''' account. [https://support.google.com/mail/answer/1173270?hl=en Here google explains]
how to create an application specific password which can be used with the booking system. If you try to use your normal googlemail password Google might block the emails from sending and treat them as spam.
If Emails are not sending please check your "Descriptive Sending Email Address", it cannot be a word only. The pop up help for the setting
has examples of valid formats.