first of all: I could see all of my email folders, they were all checked in IMAP subscription. I think I didn't make myself clear. I could see the sent-mail folder but it was empty and I couldn't copy or move messages in it.
I was expecting that somebody would tell me that the problem was with the imap server. To my mind, I don't think so. The courierimap was installed with a default Maildir configuration and about 20 people have been using it for 4 years with Kmail, Outlook and Horde(webmail). Problems appeared when everybody migrated from Kmail to Evolution (we had some different issues with kmail, nothing to do with this problem).
But I don't have time to discus about this or prove that the problem is not related to the imap server because my goal is to get rid of this problem. So I decided to migrate from courier to dovecote. I did it yesterday and it seems that the problem is solved. All folders can be accessed and all messages are visible.
Again, my goal was to get rid of this problem and it seems that this problem is solved for me. But I still think that this is a bug in Evolution and not using courierimap is not a solution. Anyway I won't be able to furnish more info about the problem because I don't use courierimap any more.
Thank you for your help, I really appreciated it.