As Microsoft continues work to migrate its 400 million Outlook.com users to a new infrastructure and UI, it has hit a roadblock involving Windows Live Mail users.
Microsoft plans to begin migrating all Windows Live Mail users' Outlook.com to the new Office 365 backend infrastructure starting June 15. But because Windows Live Mail 2012 client doesn't support the new required synchronization protocols, customers using that configuration won't be able to send or receive Outlook.com email from Windows Live Mail 2012 once their accounts are upgraded.
Windows Live Mail was part of Microsoft's Windows Live Essentials (now known as "Windows Essentials") application bundle. Other elements in that bundle include Photo Gallery, Movie Maker, Windows Live Writer and OneDrive.
Even though Microsoft hasn't updated Windows Live Mail in years, quite a few people still use it. According to Microsoft there are 3.5 million users with Outlook.com accounts connected to Windows Live Mail. (Others have their Live Mail accounts connected to Gmail and/or other clients.
From Microsoft's blog post on May 5 about the migration problem:
"If you currently use Windows Live Mail 2012 to connect to Outlook.com, we recommend that you switch to the Mail app on Windows (before your account is upgraded). This email app is used by the vast majority of Outlook.com users today. It is designed to work with the new Outlook.com, offers a more modern and touch-friendly interface, and integrates elegantly with the Windows platform. Like Windows Live Mail 2012, the Mail app on Windows is free."
The suggested Mail app mentioned above is bundled with Windows 8/8.1 and Windows 10. Users can just add their accounts to that Mail client. Another option for those affected is to starting using Outlook.com in a Web browser. This is basically the only option for those still running Windows 7 (other than upgrading to Windows 8/10.)
Microsoft also plans to offer affected users another option. Microsoft will provide those Live Mail/Outlook.com users affected by the synchronization issue with a free one-year subscription to Office 365 personal and use Outlook 2016 to connect to their accounts, if they'd prefer to go that route.
More details about the migration issue and options are in this Microsoft support article.
Update: Just a quick note: This issue isn't going to postpone further the planned end date of Microsoft's Outlook.com migration. That date remains "end of summer," most likely by August.