Currently it's impossible to configure an Outlook.com account as SMTP server.
Not only does the current mailer plug-in configuration miss configuration options for providing STARTTLS, neither does it provide any wizards whatsoever for any of the popular public SMTP provider (Microsoft, Google, GMX, Web.de etc.), helping the user in configuring mailer plug-in configuration by pre-setting provider specific values and inquiring only the necessary information in a doubtless manner.
Finally: showing the exception message alone when testing the e-mail configuration is not sufficient for hunting down bugs. The full SMTP message sent should be displayed as well.