The default is “automatic”. That means the app will use whatever the server tells it the Sent folder is. If an email provider doesn’t support this feature, or, more common, supports it but doesn’t designate a Sent folder, the app will not use a Sent folder. This is displayed as “Automatic (None)” in settings.
We’re currently working on rewriting the account setup code. The new version will ask the user to configure special folders if the server doesn’t assign all special folder roles. (Users will still be able to select “None” if they wish not to use a particular special folder).
You can already test this by becoming a beta user.
The settings screen is currently using a framework that is not very flexible. It’s not trivial to display an additional warning message. We’ll keep the suggestion in mind for when we rewrite the settings screens (which is necessary for a couple of other reasons).