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This configuration is not related to ClosePlan or People.ai, the customer is responsible and it should be actually in place already, because its highly likely that customer’s SFDC org is sending email out emails on behalf of users. Unfortunately, not every company knows that such configuration should be provided…
To setup SPF records, please follow steps in the official Salesforce article:
https://help.salesforce.com/articleView?id=000315520
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