All domains, regardless if they send email, should include a Sender Policy Framework (SPF) record. SPF is a widely adopted mechanism that identifies legitimate sending IP addresses and is taken into account by mailbox providers when treating received email. SPF provides senders better control and security around the domains they own.
If you have a domain that is not used for sending email, such as a defensively registered domain, the following SPF record should be used in the DNS:
v=spf1 -all
For example:
email.yourdomain.com. IN TXT "v=spf1 -all"
