Spam is the biggest problem you can face with regards to email marketing limitations. Many companies send the email messages to only their registered customers but there are few companies which just send the message to an indifferent number of unknown customers, known as spam. So internet system administrators always use the terminology abuse of net but never think about abuse on net. How will you like to get the mails you are never interested in? It is very difficult to distinguish between legal or spam emailing and so customers cannot take any actions against them.