[exim] how to config SSL

Hi all

I bought a SSL certificate (comodo),and config SSL as below:
------------------------------------------------------------------
tls_advertise_hosts = *
tls_require_ciphers = SSLv3:TLS
tls_verify_certificates = /etc/ssl/comodo/CA.crt
tls_certificate = /etc/ssl/comodo/server.crt
tls_privatekey = /etc/ssl/comodo/server.key
#tls_certificate = /etc/ssl/server.crt
#tls_privatekey = /etc/ssl/server.key
-------------------------------------------------------
test SSL
$ openssl s_client -connect mail.saybot.com:smtps

CONNECTED(00000003)
depth=0 /OU=Domain Control Validated/OU=PositiveSSL/CN=mail.saybot.com
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 /OU=Domain Control Validated/OU=PositiveSSL/CN=mail.saybot.com
verify error:num=27:certificate not trusted
verify return:1
depth=0 /OU=Domain Control Validated/OU=PositiveSSL/CN=mail.saybot.com
verify error:num=21:unable to verify the first certificate
verify return:1
exim vesion:4.76

Anyone made such successful configure?