To authorize Rackspace to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used for Rackspace is shown below.
For more information on SPF click on the button below.
Rackspace support DKIM signing for your custom domain. For more information and instructions on how to enable DKIM for your domain please click on the button below.
Create a free OnDMARC account to test your configuration.