>> Прежде чем выполнять запросы, надо выбрать базу. Типа: use postfix;
>  поспешил ответить, а пока редактировал ответ Вы ответили.
> поправил, посмотрите предыдущее сообщение

Спасибо за подсказку. Поправил файлы

/etc/postfix/alias.conf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
table = alias
select_field = goto
where_field = address
additional_conditions = and active = '1'
query = SELECT goto FROM alias WHERE address='%s' AND active = '1'

/etc/postfix/domain.conf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
table = domain
select_field = domain
where_field = domain
additional_conditions = and active = '1' and backupmx = '0'
query = SELECT domain FROM domain WHERE domain='%s' AND backupmx='0' AND active='1'

/etc/postfix/mailbox.conf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
table = mailbox
select_field = maildir
where_field = username
additional_conditions = and active = '1'
query = SELECT maildir FROM mailbox WHERE username='%s' AND active = '1'

/etc/postfix/quota.conf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
table = mailbox
select_field = quota
where_field = username
additional_conditions = and active = '1'
query = SELECT quota FROM mailbox WHERE username='%s' AND active = '1'

/etc/postfix/sender.conf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
table = mailbox
select_field = username
where_field = username
additional_conditions = and active = '1'
query = SELECT username FROM mailbox WHERE username='%s' AND active = '1'

В базе всё создаётся, в базе username записан как name@domain.ru, в файле dovecot-mysql.conf в запросе стоит username = '%u' это правильно или нужно username = '%u@%d' ?