To authorize Creamailer to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used for Creamailer is shown below.
For information on how to set up DKIM for Creamailer please contact their support team. You may have to upgrade your package with them if the one you already use does not include domain authentication.
Create a free OnDMARC account to test your configuration.