KMail 1.4 can't change the sender

Hello, I' ve a little trouble with kmail1.4 under kde3.0. If I write a mail and change the identity to an other then standard and hit the sendbutton i always get the message "unknow sender" or similar. This message is ok, because kmail tried to send the mail with an other sender as standard but takes always the standard account. i.e. I like to send a mail with my t-online-account, so I choose the t-online-identity, but kmail tried to send this mail with web.de as standard. How can I change this, I know it must be possible, but I'm not able to find it. Michael