By default, Windows 10's Mail app will automatically add a "Sent from Mail for Windows 10" signature to your outgoing emails. If you would like to remove this from your email footers, you can customize your email signature in the Mail app settings.
If you are using Mail app to manage multiple email accounts (Gmail, Outlook, Yahoo...), you can customize your signatures for each of your accounts from this settings menu.
Open the Mail app, click on the Cog icon and go to Settings > Options. Click on the drop-down menu displayed at the top of the Options panel and select the desired email account:
Next, scroll to the Signature section and clear the default Sent from Mail for Windows 10 signature:
Type your new signature and close the Options panel when done.
Moving forward, the Mail app will automatically apply the new signature to your email messages.
N.B. If you would like to send emails without a signature, you simply have to toggle the Use an email signature switch Off.