There can be different reasons for this:
Booking automation emails are blocked by your email server.
The email is blocked by the receiving server because of the security settings.
There is a mismatch between the registered email in channels (Airbnb) and the email you use to send automated emails (This happens when you use your own email address to send emails).
Possible solutions:
Log in to your email server and make sure that you give access to "less secured apps". Usually this could be found in "Security" section.
Give access to 3rd party applications.
Switch off two factor authentication in gmail
Register you own email in Booking Automation: Settings > Account > Outgoing Email
Very Important - Whenyou register your own email in booking automation, In most cases still there will be error. This needs a requirement to change the current password of your GMAIL account and update it back on beds24 account. Go to : Account > Personal Info and change your password of the gmail.
Update the new password at Booking automation> Settings > Account > Outgoing Email
If you use Gmail please go here: http://help.bookingautomation.com/articles/3072703-setting-up-your-own-outgoing-email-gmail
If you use Gmail or Hotmail email servers please go here for the general set up guide: https://www.bookingautomation.com/wiki/index.php/Outgoing_Email