The trash folder is cleared of messages when you quit KMail if this option is selected.
All messages queued in the outbox folder will be sent whenever you check your mail.
If you don't like the Composer you can use any editor. Note that the Composer window will still open and the external editor will open as soon as you type just one character in the body of the message.
This let's you configure actions for the arrival of new mail. Note that automatically fetching mail can be configured in your account settings under Receiving Mail.