please, I got the following message from you:
This problem is because your have a limitation.Don’t allow to send 2 mail in same time from same account. You can fix this problem 2 ways :
1 You can contact your hosting provider and tell to fix your problem.
2 You must go in In wp-admin -> Booking System PRO -> Calendars – > Select your calendar – > Edit calendar notifications – > and User notifications method from SMTP to SMTP 2. Then you must complete settings for SMTP 2 ( must be a different account , not same like in SMTP ).