Suggest a query

Hi friends,

I have a table in my postgre data base. In that table I have a field
called folder_name whose value will be full path of the mailbox . For
example /gomathi/ax/ay/mbox is the value of folder_name,
where mbox is the mail box. I want to find out all the mail boxes
whose parent is mbox.

Actually I can try as

Select folder_name from mail_box where folder_name like 'mbox/%' ;

but I should not hard code the folder_name as given above after
'like'. SInce folder_name comes as a parameter to the program the
value cannot be hardcoded.
Please suggest an alternative way to find out the sub folders of folder
mbox.

Regards,
gomathi