To authorize ServiceNow to send emails on your behalf. you will have to add their SPF mechanism to your SPF record or into Dynamic SPF. Their include mechanism is as follows:
​
​include:service-now.com
​
Remember, if adding to Dynamic SPF, you only need to enter the FQDN, without the include: part.
​

To configure DKIM for Service Now, please follow the article linked.
​ServiceNow KB1002273
​
​Note: You must have an active ServiceNow contract and be an authorized support user to access the article.