Oct  8 18:14:55 Debian-86-jessie-64-minimal dovecot: master: Error: service(auth): command startup failed, throttling for 60 secs
Oct  8 18:26:11 Debian-86-jessie-64-minimal dovecot: auth: Fatal: mysql: Missing value in connect string: connect
Oct  8 18:26:11 Debian-86-jessie-64-minimal dovecot: master: Error: service(auth): command startup failed, throttling for 2 secs

disable_plaintext_auth = yes
auth_mechanisms = plain login
!include auth-sql.conf.ext

driver = mysql
connect = host=127.0.0.1 dbname=<dbname> user=<user> password=<password>
password_query = SELECT email as user, password FROM virtual_users WHERE email='%u';