SPF
To authorize Marketing Cloud to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used for Marketing Cloud is shown below.
include:cust-spf.exacttarget.com
DKIM
If you have the Sender Authentication Package you will be able to enable DKIM. Please contact your account manager at Marketing Cloud for more information.
Create a free OnDMARC account to test your configuration.