Authentication plug-ins allow various plug-ins to authenticate the client. Look in plugins/auth and perldoc any modules which use authentication schemes your system supports. Then add them to config/plugins, one line per auth plugin. Depending on the particular plug-in, the additional auth plug-ins will be skipped if the last auth plugin returned OK.

Qpsmtpd currently has the following authentication plug-ins in the distribution:

• Flat file authentication (auth_flat_file)
• Vpopmail based authentication (auth_vpopmail_sql)
• LDAP authentication (auth_ldap_bind)
• Credential Validation Module (CVM) http://untroubled.org/cvm/ authentication (auth_cvm_unix_local)

There is also a plug-in to do authentication via the standard qmail interface, checkpassword. The plugin is named authcheckpassword.

If you require alternative authentication plug-ins then there are a number of community provided ones on the plugins page.

Additionally, if you wish to develop your own authentication plug-in then you can use the authentication portion of the plug-in API documentation.

This seems to be a common issue so it should probably have a page of its own, looking at FAQ, configuration, troubleshooting and the relevant API issues