If you're trying to sign in to Microsoft To Do on the web and see a 503 error, this could be related to limitations in the local network or restrictions on a corporate network.
Check with your IT admin to make sure none of these URLs are blocked:
- .microsoft.com
- .msecnd.net
- .live.com
- graph.windows.net
- graph.microsoft.com
- .office.com
- .office.net
- .office365.com
- static2.sharepointonline.com
- todo-web-endpoint.azureedge.net
- .microsoftonline.com
- .microsoft.io
- .windows.net
- microsoft-my.sharepoint.com
- microsoft.sharepoint.com
- login.microsoftonline.com