Urgent: Sending in KMail: 'Sender address does not exist' (?)

Hello all, (SuSE 8.2) I don't know what happened, KMail don't want to send any mail and write that message ('Sender address does not exist') instead. Of course, sender has address, it can be even seen in From field of composer. What's worse, when I click on first mail (and then on second, etc) in Outbox, KMail deletes it (without leaving a copy!) and mail: 'from: nobody, subject: no subject', i.e. an empty mail. Is it possible to do something without upgrade? Thank you in advance. Nikola