>946
これでどう?
http://wiki.dovecot.org/SSL/DovecotConfiguration

Client certificate verification/authentication

If you want to require clients to present a valid SSL certificate, you'll need these settings:

ssl_ca_file = /etc/ssl/ca.pem
ssl_verify_client_cert = yes

auth default {
ssl_require_client_cert = yes
#ssl_username_from_cert = yes
..
}