one thing i can think off, is open kmail got to settings>configure kmail.
now another screen should pop up, the first tab says identities, in the middle of the pop up screen your default email address should be there, double left click on it.
another screen pops up it should have 3 tabs, General Advanced signature.
In the general tab fill out your name and on the bottom line put in your email address.
Then go to the advanced tab, there will be a Reply-To address, put in your email address in there, now that is done you now need to delete all the emails you have tried to send, they will most likely sitting in the outbox ready to be sent, they need to be deleted otherwise they will stop any other emails from being sent. once they are gone start a new email and see if that will be sent.
hope this helps