Use Lightweight Directory Access Protocol (LDAP). References:
OpenLDAP
OpenLDAP Admin Guide in the openldap-guide package
openldap-guide
LDP: LDAP Linux HOWTO
LDAP Linux HOWTO
LDP: LDAP Implementation HOWTO
LDAP Implementation HOWTO
OpenLDAP, extensive use reports
Open LDAP with Courier IMAP and Postfix