Kmail 2 error: Sending messages fails with "Unknown collection"

According to at the Kmail forum this was probably an Akonadi issue:

Kmail 2 / KDE 4.8 / opensuse 12.1

When trying to send message through smtp.gmail.com (yes user, password is
OK) I get "There were problems trying to queue the message for sending:
Unknown collection."

Migrated from opensuse 11.4 with non-akonadi-kmail, but have more or less
migrated the emails and much of the settings by hand, however I was not
able to make kmail not remeber all settings.

I have all my emails in "Kmail folders" downloaded from gmail with pop3.
"Local folders" are empty.

Both "kmail folders" and "local folders" has an "outbox" folder (lower case
"o").

Here is the thread on the KDE forum:
http://forum.kde.org/viewtopic.php?f=215&t=99353&sid=86ae50f21a6642dd13c0501e4cedb678&p=214471#p214471

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