Removing an email account takes it out of Newton on all your devices, not just the one you're using. Your mail isn't deleted: it stays with your email provider, and you can add the account again later.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to Accounts and select the account.
Click the trash icon below the account list.
In Remove account?, click Remove.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap Accounts, then tap the account.
Scroll to the bottom and tap Remove account.
Tap Remove.
The prompt asks whether to remove the account "from this device", but it's removed from all your devices.
Emails you scheduled from that account are cancelled, and its read-receipt history is deleted.
For Gmail, removing the account also removes Newton's access in your Google account. Microsoft doesn't let Newton do that, so for Outlook, also remove Newton in your Microsoft account.
The mobile app can't remove your only account. Log out instead.
A team shared mailbox can't be removed from the apps. A team admin detaches it from the team in the web portal.