This section allows you to change the type, size and character set of the display fonts. Message Body sets the font for both the reader pane and the composer window. There's a separate entry for Message List - Date Field so you can choose a monospaced font for the date field for better readability.
This section allows you to change the color of the text. Recycle colors on deep quoting means that even text that's quoted more than three times will appear in color.
Show long folder list lets you toggle the long folder list on and off. The long folder list shows folders all the way down the left side of the KMail main window, which lets you see more folders than in the short view, but it takes away some space for viewing the text of the messages. With Display message sizes selected there will be another column in the Header pane that shows the messages' size in bytes. Thread list of message headers will put all the messages in a kind of tree, so the replies to a message are below that message.
With Message header threading options you can select under which conditions threads should appear open by default (i.e. not collapsed). You can still open/close threads using the +/- buttons if you don't disable that here.
With Display of Date you can choose between several date formats. The Localized Format is the one you can specify in KControl.
This section allows you to choose between several address books, whereas Traditional KMail is a very simple one and KAddressBook is an advanced one - we recommend to use KAddressBook. Note that you need the kdeutils package to use KAB.
While the Traditional KMail database is independent (i.e. you will not see addresses in KAB/KAddressBook if you only entered them in the Traditional KMail address book), KAB and KAddressBook use the same database.
In this section you can manage rules that define a message's score. If at least one rule is defined for a folder, a Score column will appear in the Headers pane of that folder. Each rule has a name, a list of folders where it is active, one or more conditions that decide whether the rule matches a message, and one or more actions that get triggered if a condition is matched. This concept is very similar to that of filters.
The most useful action is adjust score. It will increase or decrease the score of a message. The idea is that important messages have a high score and you can then use the Score column to sort your messages.
The display message action will display a dialog when you enter a folder with a matching message.