Google announced today that its Gmail service has 425 million monthly active users. That means it has blown past Hotmail for the first time, becoming the largest email service in the world.

For many years, Microsoft’s Hotmail has been the reigning champ among global email services. In July, 2011 Hotmail announced on its 15-year anniversary that it had 360 million unique users per month. Yahoo Mail used to be the clear No. 1, but it has seen customers defect to both Hotmail and Gmail. And finally Google’s much-younger service (Gmail’s invite-only beta release was in 2004) has caught up to its older rivals.

Gmail’s growth has been absolutely stunning. In January, the company said on an earnings call that it had 350 million monthly active users on Gmail, based on its own internal data. Today that number sits at 425 million monthly actives, again based on its own data with no third-party confirmation.

Web analytics company comScore, on the other hand, told us that Google has way less unique visitors and still gives the edge to Hotmail and Yahoo. ComScore’s latest numbers from May have Hotmail at No. 1 with 325 million unique visitors, Yahoo at No. 2 with 298 million users, and Gmail at No. 3 with 289 million users.

We contacted representatives for Microsoft, Yahoo, Google, and comScore to try to reconcile the numbers. Each company, in some way, provided a frustrating response.

ComScore VP Andrew Lipsman told us that the company’s data was based on global unique visitors from “home and business use,” which leaves out important things like smartphone and Internet cafe access. “There are going to be some users that are left out,” Lipsman acknowledged.

Both Yahoo and Microsoft directed us to the above comScore data as their metric for active users. We asked Microsoft and Yahoo to get more specific with their own internal numbers, but both declined and again pointed us to the latest comScore data.

When I asked a Google spokesperson why its internal numbers showed a discrepancy of more than 100 million users compared with comScore’s data, he told us that the company doesn’t comment on third-party numbers.

Using Gmail’s “internal numbers” to compare against Yahoo and Microsoft’s comScore data may not be truly fair, but these are the numbers each of the companies stand by. Therefore, these are the numbers we will use to proclaim Gmail as the number one e-mail client in all the lands.

Long live King Gmail! Until Microsoft and Yahoo cough up better stats, that is.