As part of its quest to attract users to its Gmail service, the Internet search company has introduced dozens of features, including one that, after a certain time, makes a user solve a math problem before sending an email, giving them time to rethink it. Because Google makes money every time email users click on ads, it is enhancing its email service to increase advertising and take market share away from Yahoo.

Unique visitors to Google's sites increased 32 per cent worldwide to more than 775 million last year, according to comScore, which tracks such data.

Yahoo had a 16 per cent gain to 562.6 million visitors and Microsoft had a 20 per cent increase to about 647 million visitors.

Analysts have attributed part of Google's visitor growth to email features that are being turned out at a dizzying rate by the company's Gmail Labs.

This month, Google introduced a feature to automatically download mail so users can read Gmail offline in a Web browser. That matches an existing feature in the client version of Microsoft's Outlook but when Outlook is accessed from the Internet it does not have that feature.

The off-line mail feature was announced in a press statement, but most other features to Gmail have been introduced more quietly. Engineers created and posted 34 experimental features in the seven months since Gmail Labs launched in June.

"They're able to improve the products much faster than anyone else," said Forrester Research analyst Ted Schadler.

Google said those features are for adventurous Gmail users because the rapid addition of them means they may not work smoothly or that they will last.

TESTS, REMINDERS

"Mail Goggles" helps users avoid sending regrettable email or Gchat messages, an instant messaging system, by making them pass a simple math test before sending.

Another feature alerts users who forget to upload promised attachments. And another lets users send free SMS (short message service) messages to friends via Gchat.

The new features can be found in "Labs" on the main Gmail account page in the upper right corner under "Settings".