Email is a very versatile medium. Formats range from simple text to HTML & rich media. Content can be one-size-fits-all or highly customized. Frequency can consist of fixed, frequent intervals or sporadic intervals, with transmissions occurring only when something newsworthy comes along. Sophistication (and cost) can be very low or very high.
Along with the power of email comes the abuse of email, commonly known as spam. Is spam email considered marketing? Technically, the answer is probably yes, but it is certainly not responsible email marketing. While some users fail to distinguish between permission marketing and email spam, spam is actually a major threat to legitimate email marketers, as a glut of messages could make the entire email medium less effective.