Caterbook sends emails to guests on your behalf, using our mail servers.
Historically, many people have received "spam" email, often from potentially fraudulent sources claiming (quite convincingly) to be from someone they are not.
Sender Policy Framework ("SPF") is one way that Internet Service Providers can validate that an email comes from a trusted source.
When you register your domain name (eg yourhotel.com) there are a number of records created by your hosting company, including an SPF record that shows who are your trusted email senders.
If you are using a popular email marketing platform like Mailchimp, they would advise you to add their servers to your SPF record to improve deliverability of your campaigns. It's the same for Caterbook.
You can find your own SPF record here: https://www.dmarcanalyzer.com/spf/checker/
The last statement in the SPF record describes what you want a server to do if it receives email from a server other than that listed in your record:
-all Fail – servers that aren’t listed in the SPF record are not authorized to send email, any non-compliant emails will be rejected by the recipients mail server.
~all Soft fail – If the email is received from a server that isn’t listed, the email will be accepted but flagged potentially as suspicious and might land in the recipient's spam folder (but it will be delivered).
+all Pass everything - we strongly recommend not to use this option, this tag allows any server to send email from your domain.
If you have an existing SPF record closed with "-all", then if our servers are not listed in your record, anything sent from us on your behalf is likely to be 'bounced' back to you as undeliverable.
This is likely to be the reason why you can't email Booking.com guests via Caterbook, where our other properties are able to. The recommended setting is ~all which marks mail as potentially spam, but usually providers will deliver this after taking into account other red-flag factors - known keyword spam, external 3rd party links in the mail etc.
The best solution is if you (get your IT people to) add our mail server into your existing SPF record (or create a record with our server listed in it), thereby all recipient email servers know we are a trusted source, authorised to send on your behalf.