Authentication Databases
These databases can be used as both password databases and user databases:
Passwd: System users (NSS, /etc/passwd, or similiar)
Passwd-file: /etc/passwd-like file in specified location
LDAP: Lightweight Directory Access Protocol
SQL: SQL database (PostgreSQL, MySQL, SQLite)
Dict: Dict key-value database (Redis, memcached, etc.)
VPopMail: External software used to handle virtual domains
Lua: Lua script for authentication (v2.3.0+)