Version: (using KDE KDE 3.5.1) Installed from: Compiled From Sources 'Your SMTP server does not support PLAIN. Choose a different authentication method. The server responded: "Unable to authenticate at present" This is a temporary failure. You may try again later.' A temporary error does not mean that the auth method is not supported. This is completely bogus.
I believe this is a duplicate.
Can you still reproduce this in a recent kmail version?