5. And a way to disable html and to display the mails only in plain text or simple html

I use imap, but it doesn't work.
I tried to create the folder with the exact name in SM, but the mails will not be downloaded to it.

Btw: Three other suggestions:

6. The address book could be enhanced to be able to add additional information (phone number, address, messenger nickname, etc.)

7. The ability to view and set priorities (medium, high and low) and the options "certified mail" and "private"

8. An option to see the full mail header (where also information like the used mail client are stored)

At point 6 - is it not better to integrate an LDAP connection ? There is all kind of information on it (assuming that your mail server support it)
For imap folders to be seen (downloaded) you first must subscribe to them on server.