A core GDPR requirement is to proactively inform those you interact with about your privacy arrangements. You do so by documenting and publishing a privacy policy and to reference that policy in all your communications. We advise to add a standard footer message to your emails that reference your privacy policy. Please find below a template you can adapt:

This email is sent to you by . We may hold data about you. Please read our privacy policy [add hyperlink to your privacy policy] to learn how we handle data and what your rights are. This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you are not the named addressee or an employee or agent responsible for delivering this message to the named addressee, you are hereby notified that you are not authorized to read, print, retain, copy or disseminate this message or any part of it. If you have received this email in error we request you to notify us by reply e-mail and to delete all copies of the message.