Creating a new user account and setting up KMail works - I am able to send mail from using the new user account. However, I receive a warning that the Semantic Desktop is not enabled. I was prompted to create a new outgoing account before sending the test email. Turning off the Semantic desktop and using my old identity will enable me to send email in my normal system user account.

Now this is a very weird behavior. Please file a bug on bugs.kde.org giving as many details as possible.

Oddly, it seems that restarting the Semantic Desktop has fixed the problem (or perhaps one of the steps I took were necessary).
I will make a VM installation and see if the problem exists yet again and then report a bug accordingly.

I had the same problem. Mail -> Send mail Via ... leads to the error message in the thread title

Solution is to change the Identitiy. The problem why it took me a while to figure it out: I had to add the Identity to the toolbar: Settings -> Configure Toolbar