You want to save receipts that you send into that folder regardless of what client you use.
This is a good idea even if you weren’t using SaneBox because
Sent receipts are definitely something that you want to have a historic copy of and access to from all your devices.
But your Sent folder setup is crucial for us to be able to: A) put replies to your emails in the Inbox and B) create your SaneReminders.
Here is a look at how to do this in the following email clients: