Microsoft first introduced Outlook.com back in July as a preview service designed to replace Hotmail. Starting this week, the company plans to aggressively market it as an alternative to Gmail and other web-based email services in a multimillion-dollar advertising campaign. "Outlook.com is coming out of preview," says Microsoft's Dharmesh Mehta, senior director of Outlook.com. "We'll take the preview tag off."

From today onwards Microsoft is spending millions of dollars across many countries to advertise Outlook.com on TV, radio, online, and elsewhere. The campaign will start in the US and gradually move to other countries in the coming weeks. Microsoft's new email service might be ready for the masses with the company marketing it in a big way, but it's not doing it as aggressively as the recent "Scroogled" ads. "They're actually highly targeted, there's a small number of people out there that are seeing them, but it's much smaller," explains Mehta, noting that the direct Outlook marketing will be broader.

The switch to a full service follows months of preparation by Microsoft's Outlook.com team, readying infrastructure and servers for an influx of new users. "To date we've got over 60 million people actively using the service," says Mehta. The team has been tweaking and fine tuning Outlook.com and scaling up its backend to handle the hundreds of millions of Hotmail users that will start to transfer across. "We didn't expect to grow as fast as we did in the preview," explains Mehta. "It even strained some of our systems on the backend more than we thought it would."

Out of the 60 million users, Mehta reveals that "a large chunk of those are not upgrades," but new people who have signed up for the service. A third of them are coming direct from Gmail, and the rest is a mix of users signing up for a new email address and those switching from another mail service he says. Microsoft is now preparing to take on millions more as it reaches out to existing Hotmail users and launches a massive advertising blitz to convince users to switch or upgrade.

For Hotmail users it's a simple upgrade, but Microsoft isn't forcing people to switch straight away. "We've got over 150 million gigabytes of people's inboxes and we're not gonna flip them all over night," explains Mehta. "It'll be gradual, but it will start just after the 19th, we'll send people email communications to say it's coming and of course they can choose to upgrade on their own." Microsoft expects to complete Hotmail upgrades by the summer, with the promise of an auto upgrade for the users who don't opt to switch themselves.

Existing Outlook.com users won't notice much of a change, but some upgrades are coming shortly. "You'll see much more now that we'll be out of preview, you'll see those over the months and months," says Mehta. A calendar UI upgrade and Skype integration with video calling and messaging "will be coming soon after the 19th" promises Mehta, along with an updated Android app in the near future. Microsoft will also add international domains in the near future as well to ensure users can get the name they want. "For us the preview period was getting the product ready to be available for everyone and we did a small amount of feature work, now we're going to flip the balance so you'll see a ton more coming over the next few months."