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kmail - just a little problem

Hi folks, 

I am running into a little problem with kmail in plasma.

The problem is, that the column on the very left side (the one where the 
folders like "kmail-folder" are shown)  with the the definition "name" is very, 
very big (more than 4000 pixels wide). But I can not get it smaller, and there 
appeared a scrollbar below. 

I found no way to get rid of this and get the column smaller. It cannot be 
shifted in any way. If I add the other columns like "unread", "size" and 
"general", those can be made smaller and wider.

Any idea, how this behaviour appeared and how I can get rid of this?

Thank you for reading this message and for any help.

Best regards