  With the rapid adoption of Internet as an easy way to communicate, the amount
of unsolicited e-mails, known as spam e-mails, has been growing rapidly. The
major problem of spam e-mails is the loss of productivity and a drain on IT
resources. Today, we receive spam more rapidly than the legitimate e-mails.
Initially, spam e-mails contained only textual messages which were easily
detected by the text-based spam filters. To evade such detection, spammers came
up with a new sophisticated technique called image spam. Image spam consists in
embedding the advertisement text in images rather than in the body of the
e-mail, yet the image contents are not detected by most spam filters. In this
paper, we examine the motivations and the challenges in image spam filtering
research, and we review the recent trends in combating image spam e-mails. The
review indicates that spamming is a business model and spammers are becoming
more sophisticated in their approach to adapt to all challenges, and hence,
defeating the conventional spam filtering technologies. Therefore, image spam
detection techniques should be scalable and adaptable to meet the future
tactics of the spammers.
