Downgrade dovecot to 1.0.10 give me error :

mail# /usr/local/etc/rc.d/dovecot start
Starting dovecot.
Error: Error in configuration file /usr/local/etc/dovecot.conf line 262: Unknown setting: mail_privileged_group
Fatal: Invalid configuration in /usr/local/etc/dovecot.conf

Find line 262, talk about mail_priveleged_group

solution :

change
mail_privileged_group = mail

into

mail_extra_groups = mail

and start dovecot service.

Work