SAN FRANCISCO — Microsoft is so confident it has the Internet’s best email service that it is about to spend at least $30 million to send its message across the U.S.

The barrage began Tuesday when Microsoft’s twist on email,, escalated an assault on rival services from Google Inc., Yahoo Inc., AOL Inc. and a long list of Internet service providers.

As part of the process, all users of Microsoft’s Hotmail and other email services under different domains such as will be automatically converted to by the summer, if they don’t voluntarily switch before then.

All the old messages, contacts and settings in the old inboxes will be exported to Users will be able to keep their old addresses.

Email remains a key battleground, even at a time when more people are texting each other on phones.

People still regularly check their inboxes, albeit increasingly on their smartphones. The email habit provides Internet companies a way to keep people coming back and logging in to websites, so it’s easier for email providers to track their activities. Frequent visits and personal identification are two keys to selling ads, the main way most websites make money.

That’s why Microsoft, Google and Yahoo have been retooling their email services in recent months.

To welcome new users, Microsoft is financing what it believes to be the biggest marketing blitz in the history of email. will be featured in ads running on primetime television, radio stations, websites, billboards and buses. Microsoft expects to spend somewhere between $30 million and $90 million on the Outlook campaign, which will run for at least three months.

By Microsoft’s own admission, Hotmail had lost the edge that once made it the world’s largest email service. That left an opening for Google to exploit when it unveiled Gmail nearly nine years ago.

The new features in Outlook include: the ability to send massive files, including hundreds of photos, in a single email; address books that automatically update new contact information that connects posts on Facebook, Twitter and LinkedIn; and about 60 percent fewer ads than Hotmail.

None of these features are revolutionary. Google already has a new version of Gmail that also allows for larger files to be sent in a single email. And address books in Gmail already fetch new contact information posted on Google Plus, although it doesn’t yet mine Facebook, Twitter and LinkedIn.