MOUNTAIN VIEW, California: Few areas of the internet have escaped the all-pervasive tentacles of search titan Google, but the company is now planning to enter the last domain where its rivals Yahoo and Microsoft still hold sway: the Inbox.

Yahoo Mail and Microsoft's Hotmail have over 250 million users each worldwide according to comScore, while Google's Gmail, launched in 2004, has 100m, but is growing by around 40% each year.

Recent innovations to Gmail include the addition of links to services like Google Docs and Google Calendar, with some observers seeing the move as a prelude to a possible challenge to Microsoft Office.

Other improvements include the ability to send voicemail, video and text messages via Gmail, and the addition of a task list.

Says Michael Gartenberg, a strategist at technology information specialists Jupitermedia: "Google has redefined email, Microsoft and Yahoo are starting to emphasise it once again, but it's been hard for them to keep up with Google's improvements."

Last month, Yahoo launched a "smarter Inbox" replicating the format of social networking websites.

Data sourced from Financial Times; additional content by WARC staff