Пробовал на Debian linux.
Стоит postgresql 8.0.4.
Нужно править конфиг (возможно, и для 7-й версии).
в authpgsqlrc нет отношения c - добавить
                        'disableimap' as options              \
                        from mailusers c                \
                        where login = '$(local_part)'||'@'||'$(domain)';

В /etc/postfix/mailbox.pgsql
Поправить отношение c. Т.е. должно быть:
query = SELECT substring(c.login from 1 for 1)||'/'||c.login||'/Maildir/' from mailusers c where login = '%u'||'@'||'%d';

postfix=> SELECT login as username,'' as cryptpw,pass as clearpw,506 as uid,506 as gid,'/mailboxes/'||substring(c.login from 1 for 1)||'/'||c.login as home,'' as maildir,30 as quota,fullname,'disableimap' as options from mailusers c where login = 'vvvua'||'@'||'mydomain.ltd';
        username        | cryptpw |  clearpw  | uid | gid |                home                 | maildir | quota | fullname |   options
------------------------+---------+-----------+-----+-----+-------------------------------------+---------+-------+----------+-------------
vvvua@mydomain.ltd |         | testpass | 506 | 506 | /mailboxes/v/vvvua@mydomain.ltd |       |
30 | VS       | disableimap
(1 запись)

postfix=>