As of August 2017, we are removing support for SMTP email clients which only use AUTH PLAIN to authenticate with Outlook.com servers.
How does this affect you?
Outlook.com allows users to authenticate using AUTH LOGIN, which most email clients support. Modern email clients will use AUTH LOGIN if told to do so by Outlook.com servers, without the need to change any settings on the email client.
While unlikely, some applications or older hardware (such as printers or other devices) that use Outlook.com email accounts to send email could experience issues because of this change, depending on the design or settings of the application or device. You'll need to consult the makers of such applications or devices if you have trouble sending email from your Outlook.com account from those devices.
You can also try the Microsoft Virtual Agent.