This is probably simple but....

Have just re-installed kubuntu and now when I send an email I get "E-mail sending failed: Failed to transport message. The server did not accept the sender address "bill@bill-vostro-200".
The server responded: "#5.1.8 Domain of sender address does not exist"

"bill@bill-vostro-200" is the name of my computer - how can I change 'Sender Address'

Open KMail > Settings > Configure KMail > Identities. Select your Identity > Modify. Fill in your email address, as well as any aliases you have for that address.

Thanks, that worked. I had already done as sugestedbut it didn't seem to work. I then rebooted the computer and now its working.