[Dovecot] External auth

Hello,

I saw a lot of tutorials about setting postfix with dovecot sasl but I would like more.

I want to set a SSO system with roundcube on dovecot.

I would like to know how to make dovecot work with sasl or saml, not as a provider but as a service plugging on an external provider.

I saw there is an EXTERNAL auth mechanism but I didn't find any doc about it.

Can you help me ?

Thanks.