Version: 1.9.6 (using KDE 3.5.6, Kubuntu (feisty) 4:3.5.6-0ubuntu14)
Compiler: Target: i486-linux-gnu
OS: Linux (i686) release 2.6.20-16-generic
I set a filter: settings->configure filters
My (only one) dummy filter looks like this:
filter criteria: match all the following
<size> > 1 byte
<size> > 2 byte
set transport to: the name of one of my sending accounts (differing from the default one)
Apply this filter to sent messages
When sending the X-KMail-Transport: header gets overridden correctly, but the mail is sent via the default SMTP server. This is why I think applying the filters after sending the mail (at least) partially renders the feature useless.
By the way the checkbox named properly: "Apply this filter to sent messages", as stated above there is reason to name this checkbox e.g. "Apply this filter to outgoing mails", and act accordingly.
By the way my favourite MUA is still KMail, so thank you for providing this nice software.
*** This bug has been marked as a duplicate of 48938 ***