Enable vanity mail

Content

Hello,

We need to enable vanity email for our client. To do this we followed the steps 1 and 2 (Step 1 : Create Profile Option Step 2 : Enable Vanity Email) in the document 21B_Vanity-Email_Oracle-Recruiting. In the document a further step is explained, Step 3: Configure Vanity Email:
"To ensure that your outbound email is delivered successfully to your external
candidates, you must set up a Sender Policy Framework (SPF) policy on your
domain ".

Could anyone help us understand what this last step is for? We can't understand what it is for considering that we can already see the vanity email changed without having to follow the last step.