да, конечно вы правы, довекот аутентификацией занимается и лог довекота, который не может рахешить пароль.

вот имеющиеся пакеты:
dovecot-common 1:2.1.7-1ubuntu2 all
dovecot-core 1:2.2.24-1 amd64 secure
dovecot-imapd 1:2.2.24-1 amd64 secure
dovecot-mysql 1:2.2.24-1 amd64 secure
dovecot-pop3d 1:2.2.24-1 amd64

вот пакеты нового сервера
dovecot-core 1:2.2.22-1ubuntu2.4 amd64
dovecot-imapd 1:2.2.22-1ubuntu2.4 amd64
dovecot-lmtpd 1:2.2.22-1ubuntu2.4 amd64
dovecot-mysql 1:2.2.22-1ubuntu2.4

dovecot-sql.conf на сущ.сервере
driver = mysql
connect = host=localhost dbname=postfix user=postfix password=postfix
default_pass_scheme = MD5-CRYPT
password_query = SELECT username as user, password, 8 as uid, 8 as gid FROM mailbox WHERE username = '%u'
user_query = SELECT '/var/mail/%u' as home, 'mbox:~/mail:INBOX=/var/mail/%n' as mail FROM mailbox WHERE username = '%u' AND active = '1'

новый сервак:
driver = mysql
connect = host=127.0.0.1 dbname=postfix user=postfix password=postfix
default_pass_scheme = MD5-CRYPT
password_query = SELECT username as user, password, 8 as uid, 8 as gid FROM mailbox WHERE username = '%u'
user_query = SELECT '/var/mail/%u' as home, 'mbox:~/mail:INBOX=/var/mail/%n' as mail FROM mailbox WHERE username = '%u' AND active = '1'