Offering proof that open source companies can make money from paid subscribers, Zimbra on Wednesday said it has passed the 40 million mark for paid mailbox users.

Zimbra, purchased by Yahoo for $350 million in 2007, has seen its paid mailboxes skyrocket from 20 million early this year, according to the company.

Zimbra's meteoric rise in the e-mail market is built in part on deals it has signed with Comcast, as well as with several other service providers, some observers believe.

In more encouraging news, Zimbra now has 31.4 million unique monthly visitors in the United States in January, edging past Google's Gmail, which had 31.2 million, according to ComScore.

Google recently reported that it had 10 million users for both the free and paid versions of its Google Apps business bundle, but that still puts it significantly behind Zimbra.

Zimbra's dramatic improvement could prove strategically important for its beleaguered owner, Yahoo. In a presentation last week, new Yahoo CEO Carol Bartz said that Yahoo Mail is becoming increasingly important to the company.

According to numbers from Hitwise, which analyzes Web traffic trends, Yahoo Mail is responsible for some 36% of Yahoo's overall traffic, with the company's home page accounting for another 25%, and the Yahoo Search page kicking in another 12%.

Hitwise figures also show that Yahoo Mail is still the leader among Web-mail providers. February numbers show Yahoo Mail owning 56% of the U.S. market, with Windows Live Mail owning 19% and Gmail in third place with 11%.

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