The issues seems to be caused (in Windows 10 at least) by dismissing new mail notifications in the Windows Notification area (or in the noification popup) instead of opening the Mail app to check the mail. People have been reporting this, complaining about it, and seeking solutions since windows 8 but it’s still an issue with apparently no clear solution. Sometimes, the count may be higher than 1 but the most common scenario is just 1 phantom message. Periodically, the Mail app seems to get stuck displaying 1 unread message when in fact all messages have been read, something that has come to be known as the “phantom message”. I use Microsoft Outlook as my primary email client but for less urgent things (things that don’t necessarily need to have immediate attention) I also have some accounts set up in the Windows 10 Mail app.