Mutual auth and MS Outlook

I am trying to get Dovecot IMAP and Outlook to talk to each other with SSL and client certificates enabled. In Dovecot, I have the following options enabled:

ssl_ca = ...

ssl_verify_client_cert = yes

auth_ssl_require_client_cert = yes

auth_ssl_username_from_cert = yes

when I try to connect with Outlook, I get:

May 12 08:07:50 mail dovecot: imap-login: Disconnected (client didn't send a cert): user=<>, method=PLAIN, rip=192.168.1.245, lip=192.168.2.5, TLS: Disconnected, session=<is7gpa+Im97AqAH1>

But when I use openssl directly with the same certificates it seems to pass the certificate in just fine. Does anybody have any ideas?

Thanks in advance.