Step 1: Login to the Plesk panel with your admin login credentials.
Step 2: Click the Tools and Settings section and select the Mail server settings under Mail tab.
Step 3: In the SPF spam protection section, enable the ‘Enable SPF spam protection to check incoming mail’ check box.
Step 4: In the ‘SPF checking mode’ drop-down box, you can select what to do when the email fails in the local and guess rules.
Step 5: In the ‘SPF local rules’, provide rules similar to this.
Eg., include:spf.trusted-forwarder.org
Step 6: You can also set the rules in the ‘guess rules’.
Eg., V=spf1 +a +mx –all
Step 7: Click Ok to save the change that has been made here.