Kmail does not send

KMail receives ok but does not send.
Thunderbird (on Ubuntu desktop) and Outlook (on Windows) desktop both send OK
Error message is ""Transport "sendmail" is invalid""
My standard settings are
SMTP [smtp.tiscali.co.uk] e-mail and account name is <email address hidden>
Outlook & Thunderbird use port 25
"use secure connection?" answer is NO
I have a desktop PC connecting to the internet via a broadband router
What is wrong please?