I'm trying to set up an email account that uses no encryption so I configured it in Thunderbird account settings as
Connection security: None
Authentication method: Password, transmitted insecurely
but when I try to send an email I get this error dialog:

Sending of message failed
The Kerberos/GSSAPI ticket was not accepted by the SMTP server ...
Please check that you are logged in to the Kerberos/GSSAPI realm.

Why?

Thunderbird 12.0.1 Kubuntu 11.10