The increasing volume of spam email messages is driving users to look for alternative media such as instant messaging and low cost VoIP calls, analyst firm IDC predicts in a new study.

"Spam volumes are growing faster than expected due to the success of image-based spam in bypassing antispam filters and of email sender identity spoofing in getting higher response rates," said Mark Levitt, program vice president for IDC's Collaborative Computing and Enterprise Workplace research.

"Instant messaging, joined by free and low-cost VoIP calling, will result in slower email growth, especially among teens and young adults."

IDC projects that 2007 email levels will reach 97bn messages, totalling 5 exabytes of data.

Spam is expected to be largest single category of that pool with 40bn messages. Unsolicited email also for the first time ever will exceed person-to-person email messages, the firm predicted.