dovecot_sqlite.conf

driver = sqlite
connect = /etc/mailbox.sqlite
password_query = SELECT password, (username || '@' || domain) AS user FROM mailbox WHERE username = '%n' AND domain = '%d'
user_query = SELECT '/var/vmail/%d/%n' AS mail, 999 AS uid, 8 AS gid FROM mailbox WHERE username = '%n' AND domain = '%d' AND active = '1'
iterate_query = SELECT username, domain FROM mailbox

maildb.sql

CREATE TABLE mailbox (
        username varchar(255) NOT NULL,
        domain varchar(255) NOT NULL,
        password varchar(255) NOT NULL,
        active tinyint(1) NOT NULL default '1'
);

vmailbox_sqlite.cf

dbpath = /etc/mailbox.sqlite
query = SELECT '%d/%u/' FROM mailbox WHERE username = '%u' AND domain = '%d'