Courier Authentication Library
  Download: https://www.courier-mta.org/download.html#authlib
  Overview
  The Courier authentication library provides authentication
  services for other Courier applications. In this context, the
  term "authentication" refers to the following functions:
  
    - Take a userid or a loginid, and a password. Determine
    whether the loginid and the password are valid.
- Given a userid, obtain the following information about the
    userid:
      
        - The account's home directory.
- The numeric system userid and groupid that owns all
        files associated with this account.
- The location of the account's maildir.
- Any maildir quota defined for this account. See the
        Courier documentation for more information on maildir
        quotas.
- Other miscellaneous account-specific options.
 
- Change the password associated with a loginid.
- Obtain a complete list of all login ids.
The Courier authentication library provides alternative
  implementations of these authentication services:
  
    - Use the traditional system password files:
    /etc/passwdand/etc/shadow, possibly
    in conjunction with the PAM library.
- Maintain all this information in a GDBM or a DB database.
    The GDBM or the DB database is compiled from plain text files.
    Perl scripts provide a simple interface for creating and
    editing the authentication information, then a script compiles
    the plain text files into a database.
- Use an LDAP server for authentication.
- Use a table in a MySQL database for authentication.
- Use a table in a PostgreSQL database for
    authentication.
- Use a table in an SQLite file for authentication.
All Courier components that use this authentication library,
  therefore, will be able to authenticate E-mail accounts using any
  of the above methods.
  Additional information can be found in the INSTALL file.
  The authentication API is documented.
  Further resources
  Subscribe to the courier-users
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  http://lists.sourceforge.net/mailman/listinfo/courier-users
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