Sent folder to show all emails ever sent
If you send an email and once you get a response, you archive it, you won't find it on the "Sent" folder.
Sometimes I search emails based on what I've sent, and the first stop is the "Sent" folder. Given that I archive all emails that are not relevant anymore, the Sent folder is a mixture of Sent emails with no response and non archived replied emails.
Ideally the sent folder should show all emails I've ever sent, no matter where is currently stored (either archive, inbox or any other folder).