The latest e-mail virus to clog in-boxes is expected to cost companies up to $250 million in lost productivity and technical support expenses, a security expert said Wednesday.

The MyDoom worm that is spreading across the Internet is more costly than the recent So Big or MS Blast viruses that were launched last summer because it leaves behind a "Trojan horse," said John Pescatore, vice president of Internet security for Gartner Inc.

Read more: CNN.