SPF or Sender Policy Framework is a way for a receiving system to check that the sending server has been validated to send email. This is done by checking that the sender's ID (IP address) is valid. For this process to function the receiving server checks the domain's SPF entry using DNS and then compares the result with the sender's IP.
If you would like to add support for SPF to your domain accounts then you should create the TXT record similar to the following:
your.domain. IN TXT "v=spf1 include:_spf.privaterelay.com a mx ~all"
The ‘include:_spf.privaterelay.com’ entry will return a list of all addresses currently used by PrivateRelay for sending email. It is done this way so that when we change addresses a modification of your domain DNS is not required as they are automatically included.