-f will replace the server name with the email address you enter as the return path. Who it's sent from will still be whatever you enter in the From: field. Easy - give it a try.
Just make two test scripts, load each one once to send an email, then compare the headers.
mail('recipient email address', 'subject', 'message', null, '-email@example.com');
mail('recipient email address', 'subject', 'message');
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