After a user has been successfully authenticated, Dovecot looks up the user's userdb information. The userdb lookup is also done by LDA to find out how to deliver mails for the user.
The user database lookup returns these fields:
uid: User's UID (UNIX user ID), overrides the global mail_uid setting.
gid: User's GID (UNIX group ID), overrides the global mail_gid setting.
mail: Mail location, overrides the global mail_location setting.
Optional extra fields which are passed to the IMAP/POP3 process in environment variables.
Currently supported user databases are:
Passwd: System users (NSS, /etc/passwd, or similiar)
Passwd-file: /etc/passwd-like file in specified location
NSS: Name Service Switch
LDAP: Lightweight Directory Access Protocol
SQL: SQL database (PostgreSQL, MySQL, SQLite)
Dict: Dict key-value database (Redis, memcached, etc.)
Static: Userdb information generated from a given template
VPopMail: External software used to handle virtual domains
Prefetch: This assumes that the passdb already returned also all the required user database information