[Dovecot] Dovecot LDAP schema?

Hi,

does dovecot define its own LDAP schema or should I create my own one?

(I want to provide IMAP boxes for virtual users that do not have a Unix account on the mail server and thus do not want to use the plain uid/gid entries of the posix account objectclass to avoid confusion and accidently giving access to accounts.)

regards Hadmut