In order for email avoid security alerts or refusal of emails sent through SFDC on behalf of any user, company needs to configure proper SPF DNS records.
This configuration is not related to ClosePlan or People.ai, the customer is responsible and it should be actually in place already, because its likely that customer’s SFDC org is sending email on behalf of users.
To setup SPF records, please follow steps in the official Salesforce article:
https://help.salesforce.com/articleView?id=000315520