This configuration is optional and is not directly related to ClosePlan or People.ai.
Salesforce often sends emails on behalf of users, such as email alert workflows or custom Apex initiated notifications. This also applies to all email notifications sent by the ClosePlan application.
In order to avoid security alerts or even refusal of such emails, proper SPF records should be configured in your DNS settings.
To setup SPF records, please follow the steps in this official Salesforce article: