How do I authorise my CRM or booking system to send as my domain?

Every service that sends mail on your behalf has to be authorised, or DMARC enforcement will start rejecting it:

• Find them all first - invoicing, bookings, CRM, marketing, support desk, e-commerce receipts. Aggregate reports list them.
• Add each to SPF - usually a documented include, watching the ten-lookup limit.
• Publish their DKIM key - the provider gives you a selector and a record to add.
• Check alignment - a service that sends from its own domain rather than yours will still fail DMARC even with SPF and DKIM passing.

Do this before you move to enforcement, not after the receipts stop arriving.