Microsoft is testing a new version of Outlook that will charge subscribers $3.99 a month to access an ad-free inbox and up to five custom email addresses, among other enhanced features. The trial, called Outlook Premium, was first spotted by Thurrott and is currently available only to those who request an invitation.

According to an FAQ page, users who get an invitation will receive a welcome email message about five days after signing up. They can then select from among several proposed email domain names for their customized addresses, which can be used alongside existing addresses to send messages and sign into other Microsoft services.

The landing page for the trial service mentions the ad-free inbox and five custom addresses, and also offers one year of free service before the $3.99 per month charge kicks in. The public version of Outlook.com only shows ads on emails from people who aren't in your contact list, and Microsoft offers users a paid option to remove ads completely for $19.95 a year.

Among others, those ad-free features helped Outlook.com earn a PCMag Editor's Choice award, though it remains less popular than Google's Gmail service. The new trial, along with app integration and other recent enhancements, indicates that Microsoft is committed to the platform, or at least committed to monetizing it.

Microsoft has not made a formal announcement about the Outlook Premium trial. A spokesperson told TechCrunch that "Outlook.com Premium is currently a small pilot program. We're always investigating new features based on the wants and needs of our customers, and we have nothing more to share at this time."