The Cost Calculator Builder PRO for WordPress is vulnerable to arbitrary email sending vulnerability in versions up to, and including, 3.1.75. This is due to insufficient limitations on the email recipient and the content in the 'send_pdf' and the 'send_pdf_front' functions which are reachable via AJAX. This makes it possible for unauthenticated attackers to send emails with any content to any recipient.
https://docs.stylemixthemes.com/cost-calculator-builder/changelog-1/changelog-pro-version