Last July Microsoft released a preview of Outlook.com, the company’s new web-based e-mail service designed to compete directly with Gmail. In the six months that have followed, 60 million people have signed up for Outlook.com according to a new post on the official Outlook Blog.

Microsoft said they were humbled by the fast pace of adoption and have received lots of feedback and made many improvements during that time. Redmond also used the blog post to announce the service is coming out of preview and is available for anyone to try.

Microsoft is launching a massive three-month marketing campaign to help spread the word. The company believes it will be the biggest e-mail marketing blitz ever as they will run spots on primetime television, radio stations, websites, billboards and even buses. When it’s all said and done, Microsoft could spend upwards of $90 million on the Outlook campaign.

The new ads will run alongside Microsoft’s current “Scroogled” campaign against Gmail. Redmond is using that campaign to highlight the fact that Google scans users’ e-mails to find relevant advertisements to place alongside messages.

Dharmesh Mehta, Outlook.com’s senior director, said the Gmail Scroogled ads are designed to be educational while the Outlook campaign will be motivational.

Furthermore, all Hotmail e-mail users and those under different domains like MSN.com will be automatically converted to Outlook.com in the coming months. More than 250 million users will be moved to the new service by the end of the summer, we are told.