When you forward an email to others or copy new people to an email thread, review all the content in the entire email and make sure the information contained in it is suitable for everyone. It is very easy to forward emails to others, not realizing there is highly sensitive information in the bottom of the email that people should not have access to.
To learn more, check out this OUCH! newsletter: https://securingthehuman.sans.org/newsletters/ouch/issues/OUCH-201609_en.pdf
More cybersecurity education available here: https://prezi.com/zkrdursavbgu/