You can only use the To Do app with an account that is hosted by Microsoft, such as outlook.com, Hotmail.com, live.com or an account hosted by Microsoft but using a custom domain.
To see your Braintoss messages in your To Do app, you need to turn on Flagged email in the Settings of your Microsoft To Do app. You can flag your Braintoss messages manually or automatically. Once you have flagged your messages, they will appear in your Microsoft To Do app.
How to flag your Braintoss messages automatically? Send a note with Braintoss with the message “set flag on”. Your Braintoss message will be automatically flagged, and you will see your messages in your Microsoft To Do app. If you want to disable this at a later stage, you need to send a note with “set flag off”.
If your Braintoss messages land in your spam box, you need to move them to your inbox first or
If you do want to flag it manually (and not automatically), please see below video how to flag your messages manually.