We recently set up an OpenERP system as well as a mailserver based on postfixAdmin. In order to authenticate OpenERP users I've developed a simple app for OpenERP to authenticate against an arbitrary mySQL database. Currently, only the crypt() password method is supported, but database, table and column details can be configured in the company settings.
The module is heavily based on the LDAP integration for OpenERP and works the exact same way. That is, upon first login, a OpenERP user with an empty password is generated and a template user is copied. If configured correctly, postfixadmin's E-Mail address will be set for the new user.
You can find the details (and the code!) in my mercurial repository at http://www.aboehler.at/hg/auth_mysql