Sent. That said, it is crucial to correctly detect those folders upon setup - otherwise parts of the user interface will not work as expected. It turned out, detecting those folders is quite a challenge since different mail providers adhere to different naming schemes. However, in the latest version of Mail most of the folders should be recognized correctly most of the time.
Sentfolder. I fixed this in the latest version of elementary Mail and you can now expect all sent mails to be stored correctly in the Sent folder - regardles off your mail provider. Please note: This was a non-issue for Office 365 and Gmail users.
System Settings > Online Accountsnow does verify your IMAP/SMTP credentials before they got stored. So now you can be sure your mail account works correctly in elementary Mail, once the setup in Online Accounts is completed. This fixes a bunch of issues where users did not see any mails at all - even though Online Accounts (falsely) claimed everything is fine.