Unfortunately, good judgment, thoughtfulness, and restraint often disappear when quickly typing an email and sending it off. What you say represents who you are. Your email message may be forwarded to many recipients before its final resting place. Do not say anything in an email that you would not want published on the front page of your local newspaper or produced in a court proceeding. Email is not private and can usually be found even if it has been deleted.
Email can be and has been subpoenaed.
In a face to face conversation we pick up on cues such as body language, dress, diction, dialect, facial expression, and facility with language. Your email correspondent does not have those cues. Subtle language, wit, and irony often get misconstrued in email. You can not assume anything about an email correspondent's location, frame of mind, profession, interests, or future value to you. You must give your email receivers some context.