In order to communicate through the Internet in a polite manner, a set of rules has been developed over the years, also known as netiquette. One of the rules, which is especially important for businesses is not to send unsolicited bulk emails, also known as spamming. Marketing people often see sending bulk e-mail as a convenient way of advertising, but it is experienced as very annoying by end users. Many Internet users experience clogged up Internet connections and mailboxes because of junk mail being sent to them. It should not be surprising when they will try to take counteractions against you and your organization. In many countries, like the U.S. and Canada sending junk mail is even illegal.
Under United States law, it is unlawful "to use any telephone facsimile machine, computer, or other device to send an unsolicited advertisement" to any "equipment which has the capacity (A) to transcribe text or images (or both) from an electronic signal received over a regular telephone line onto paper." The law allows individuals to sue the sender of such illegal "junk mail" for $500 per copy. (Spinner and Rayzer's guide to Nettiquette)
There are also other things that can be annoying to end users, like websites in which the back button of the browser doesn't make it return to the previous website and websites that have new windows popping open, every you click on a button or link. If you want to stay friends with the people on the other side of the net, you should act according to the netiquette, and not do the things described above.