Additional information: I just sent an e-mail from Kube with an attachment. The attachment is also not shown in the Kube Sent folder, but it is actually attached when I receive it in KMail (at a different e-mail address), and in KMail it is also attached to the originally sent message in the Sent folder.

Yes, I totally agree with 3) (easy setup). I set up an account yesterday. I'm quite an experienced user, but somehow managed to ignore the grey syntax provided by Kube for IMAP and SMTP. I fiddled around for 20 minutes before I noticed this information. Then it worked well.