This thread is important: viewtopic.php?f=62&t=4830#p25069
First: The reason that (-) is set as the default "To:" is so customers don't blindly choose the wrong queue to send to. If you use Queues to categorize and departmentalize tickets, customers can select the Queue that pertains to their situation. (Printing, computer down, email down, plumbing, maintenance). If you use queues and subqueues well, your customers can help you.
Sometimes, though, you have too many customers who just want to submit a ticket to anyone who can answer.
Your options:
1) inbound email goes to one queue
2) Restrict the Queues available globally through Custom Queues in Sysconfig
3) Restrict the Queues available to a customer via Customer Groups
4) Set a Default Queue. The Wiki says
---Setting a default queue
You can set a default queue by change the SysConfig Option "Action" in Frontend::Customer::ModuleRegistration below "Ticket"-Group to something like:
Action=CustomerTicketMessage;Subaction=StoreNew;Dest=1||Postmaster
where 1 is the Queue ID and Postmaster is the queue name.
and of course, you can change the dtl to force a hidden Queue Destination (see the link).