Something that will definitely help is setting up email authentication in Flexmail. What actually happens is that we use email authentication to send certain records along with your email. The recipient mail server can infer from this that Flexmail has permission to send out your emails. At the same time, this allows you to prove that your email messages are not forged.
To set this up, go to Home> Settings in your Flexmail account. At the bottom, click on ‘Email authentication’.