After a user has been successfully authenticated, Dovecot looks up the user's userdb information. The userdb lookup is also done by deliver to find out how to deliver mails for the user.
The user database lookup returns these fields:
uid: User's UID (UNIX user ID)
gid: User's GID (UNIX group ID)
home: Home directory. Although not required, it's highly recommended even for virtual users (see the link).
mail: Mail location (if needed to override the default 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 (v1.1+)
LDAP: Lightweight Directory Access Protocol
SQL: SQL database (PostgreSQL, MySQL, SQLite)
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