Sender Policy Framework (SPF) uses the Internet's Domain Name System (DNS) records and allows you to confirm that a third party (like Caterbook) is validated to send emails on your behalf.
This is one way of reducing the chances of email sent by the Caterbook application being tagged as spam by a recipient's email server.
Copy the following line, and have your hosting company or IT administrator add it to your SPF record in DNS:
Using Office 365 to manage your email? The following link may be useful :