Thunderbird with cyrus-imapd: Why chose client certificate?

[Frank Richter]

Hi,
I've a cyrus-imapd 2.3.12 installation with these options in imapd.conf

tls_cert_file: /etc/exim/etc/server.crt
tls_key_file:  /etc/exim/etc/server.key
tls_ca_file:   /etc/pki/tls/certs/ca-chain.crt
tls_require_cert: 0

SSL and STARTTLS are working fine.

I've imported a personal S/MIME certificate to thunderbird. When 
connecting to the IMAP server (using STARTTLS), thunderbird asks me to 
select a client cert, showing (translated from German):
    This website (!) requires a certificate for identification ...
    Chose a certificate ...

The server doesn't and shouldn't accept client certificates.
So who is wrong? My configuration, thunderbird ...

I hope somebody will enlighten me ...

Thanks,
Frank

-- 
E-Mail: Frank.Richter at hrz.tu-chemnitz.de  http://www.tu-chemnitz.de/~fri/
Work:   Computing Services,  Chemnitz University of Technology,  Germany