Version:            (using KDE KDE 3.0.8)
Installed from:    Unlisted Binary Package
OS:          Linux

This will probably happen only once/user.
After installing kde from scratch, my Defalu identity did not have an e-mail address. I composed a message, and when I tried to send I got the
"Please set the email address of the default identity. Kmail does not work without it"
I clicked ok, and added an e-mail address to the default identity using the settings dialog. (The compose window was still open)
Then I clicked "send" and all went well, except that my SMTP host gave an error that the From field had an empty e-mail address, which it did. The message in the outbox did not have an e-mail address in the From field.
It is very easy to reproduce this behaviour: just delete the e-mail address from the default identity, and then compose a message and try to send it.
In order to correct the identity in the message that was now in the outbox, I had to define another identity. The compose dialog then enabled me to chose my identity, and correctly copied the e-mail address for the dafault one.