Normally, when sending a mail, a copy of that mail is kept locally on my pc in Sent-Folder. What I'd like to realize is, that a copy of that sent mail is stored in a subfolder 'Sent' of my imap-mailbox on cyrus.
Is that possible? My MTA is postfix.
That would normally be a function of your MUA not your MTA. Any MUA with good IMAP support should allow you to configure it to store file copies, drafts, and trashed messages in IMAP (sub)folders instead of local folders. Exactly how to tell it to do that varies with the MUA.
-Pat