Version:            (using KDE KDE 3.1.2)

I think the function explained below would be helpful for many people receiving ill-formatted mail headers. (lots of web based clients use actually rather sloppy formatted headers)

ability to edit the header of an e-mail received or already sent.
useful because: 

a. some ?iso-8859-1? appears sometimes without being interpreted because some mailers send ill-formatted subjects

b. if charset isn't set to the right value by the sender
(for instance, it is needed to change options in elmrc for elm to be able to modify the charset message written. Certainly not very handy and many people forget to do so. Moreover some web based email addresses don't allow to set the charset used
(they send it as html coded which isn't correctly interpreted by text-based e-mail readers as well as by kmail)

an example:
you receive an email in russian, koi8-r encoded but whose header contains charset=iso-8859-1
you can see the content if you change the option from "auto" to "koi8-r" but you'll have to do it again everytime you look at this mail. If you change the header to koi8-r, it'll be "cured" permanently.
Of course one could do this by hand using a text editor. But it may be dangerous even if kmail is not running because of the strange names of the files (dirmail) and of the .index files. So the files may be displayed next time as unread....