Version:           неизв. (using KDE 3.5.8, Kubuntu (gutsy) 4:3.5.8-0ubuntu3.1)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.22-14-generic

For some reason SMTP X509 Certificate Authentication is not working in Kmail.
After selecting appropriate certificate in the dialog it fails with Access Denied error.
SMTP server was configured to allow delivery only if Certificate is valid.
Here is a log:

Dec  8 20:24:00 cassini postfix/smtpd[30785]: setting up TLS connection from hq[89.162.141.1]
Dec  8 20:24:00 cassini postfix/smtpd[30785]: SSL_accept:before/accept initialization
Dec  8 20:24:00 cassini postfix/smtpd[30785]: SSL_accept:error in SSLv2/v3 read client hello A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:error in SSLv2/v3 read client hello B
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 read client hello A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write server hello A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write certificate A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write key exchange A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write server done A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 flush data
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:error in SSLv3 read client certificate A
Dec  8 20:24:05 cassini last message repeated 2 times
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 read client key exchange A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:error in SSLv3 read certificate verify A
Dec  8 20:24:05 cassini last message repeated 3 times
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 read finished A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write change cipher spec A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 write finished A
Dec  8 20:24:05 cassini postfix/smtpd[30785]: SSL_accept:SSLv3 flush data
 
The same Certificate verification via IMAP work well.