Javier Soltero, corporate vice president for the Outlook team, warned that Windows Live Mail 2012 users won't be able to send or receive mail on Outlook.com, and they need to upgrade their mail app.
The recommendation put forth by Microsoft:
If you are using Windows 7, upgrade to a newer operating system and use the Mail app.
If you are using Windows 7 and you do not want to upgrade to a newer operating system, get used to checking your Outlook account via a Web browser. The Outlook.com team should also be sending you an email offer for a one-year free subscription to Office 365, so you can check out the Outlook 2016 application.
If you are using Windows 8.1 or Windows 10, switch to the built-in Mail app, navigate to “Add Account” and enter your Outlook.com email address and password. Your mail will sync up.
When will you need to upgrade? The short answer is "Before your Outlook.com account is upgraded." That means you should have your preferred upgrade method in place (Mail, web-only access, Office 365 trial) in place before June 30, 2016.
If you'd like to learn more about the Mail app, we've got you covered:
How to: Create an email account in the Windows 10 Mail App
How To: Change Email Sync Frequency in the Windows 10 Mail App
How to: Turn off Conversation View in the Windows 10 Mail App
How To: Turn On and Customize Email Notifications per Account in the Windows 10 Mail App
How To: Manage Email Folders in the Windows 10 Mail App
How to adjust Mail App Sync Settings on Windows 10
Hands-On with Windows 8.1: Mail
Windows 8 Tip: Adding Accounts in Mail App