We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/windows-live-custom-domains-invalid-credentials/196
Unfortunately Mailspring doesn't support connecting Windows Live custom domains accounts, since they've been deprecated by Microsoft.
However, since those accounts aren't much more than aliases to a regular Outlook.com account, you should be able to connect your original @outlook.com account and to set up aliases within Mailspring.
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