On a typical day, Hotmail subscribers collectively receive more than 1 billion pieces of junk e-mail. Such spam accounts for 80 percent of messages received — not including mail blocked by Hotmail’s first line of filters.

Though Hotmail develops various tools for evading spam, unwanted messages keep slipping through.

“And it’s increasing every day,” said Parul Shah, a product manager with Microsoft Corp., which runs Hotmail. “Every time Hotmail or another e-mail service provider finds a way to detect spam, the spammer immediately has a way to get around that.”