Those messages that are not sorted correctly do not have an RFC-compliant date format, they break the standards.

See http://www.ietf.org/rfc/rfc1123.txt and http://www.ietf.org/rfc/rfc822.txt

My guess would be that they are created by some badly written and broken webmail?

You can use Claws Mail's Actions feature and a script to fix these mails. See the Claws Mail FAQ Actions page at http://www.claws-mail.org/faq/index.php/Actions, the last entry in the table, 'Fix missing date', is what you need.

regards

Paul