I've found the Windows 8.1 Mail App uses the wrong 'From' address when clicking on 'mailto:' links.

With Outlook.com you can set it up to send and receive email from an external mailbox account instead of from the Outlook.com account. Basically Outlook.com acts as an email client and sends and receives from external email accounts via POP/SMTP. If I create a new email from scratch or reply to emails, everything works properly and it sends all emails from my external email address as it should. However, if I click a mailto: link, it ignores all the settings and uses my Microsoft Account (@live) email address as the 'from' address.

The settings in the Outlook.com account are set to send email from my external email address and also the settings in the Windows 8 Mail App are set to use my 'Preferred Email Address', which is one from my external email account. Other than that I can't see how to change it, so I'm thinking maybe it was overlooked when they made the Mail app?

Realistically, I think maybe mailto: links should use whatever the 'Preferred Email Address' is set to in the Mail App? Unless anyone has any ideas on a work around?