dovecot.conf :

protocols = imap pop3 imaps pop3s
disable_plaintext_auth = no
ssl_cert_file = /usr/local/etc/apache2/ssl.crt/webmail.crt
ssl_key_file = /usr/local/etc/apache2/ssl.key/webmail.key
login_process_size = 64
mail_location = mbox:~/mail/:INBOX=/var/mail/%u
mail_extra_groups = mail
verbose_proctitle = yes
first_valid_gid = 0
mbox_read_locks = fcntl
mbox_write_locks = dotlock fcntl
protocol imap {
  imap_client_workarounds = delay-newmail outlook-idle netscape-eoh tb-extra-mailbox-sep
}
protocol pop3 {
  pop3_uidl_format = %08Xu%08Xv
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
protocol lda {
  postmaster_address = postmaster@example.com
  sendmail_path = /usr/sbin/sendmail
}
auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@
auth default {
  mechanisms = plain login
  passdb pam {
  }
  passdb sql {
    args = /usr/local/etc/dovecot-sql.conf
  }
  userdb passwd {
  }
  userdb prefetch {
  }
  user = root
}
dict {
}
plugin {
}

dovecot-sql.conf:

driver = sqlite
connect = /var/db/mail/virtusers.sqlite
default_pass_scheme = PLAIN
password_query = SELECT login || '@' || domain.name AS user , password , 'maildir:/var/virtusers/'|| domain.name || '/' || login || '/Maildir' AS userdb_mail , 20000 AS userdb_uid , 20000 AS userdb_gid FROM user JOIN domain ON ( domain_id = domain.id ) WHERE login = '%n' AND domain.name = '%d' UNION SELECT login AS user , password , 'maildir:/var/virtusers/'|| domain.name ||  '/' || login || '/Maildir' AS userdb_mail , 20000 AS userdb_uid , 20000 AS userdb_gid FROM user JOIN domain ON ( domain_id = domain.id ) WHERE login = '%n' AND domain.name = 'default.domain.tld' AND not length('%d')