[otrs] Solution for a ticket state "collecting"

We have customers, which send us several mails to a ticket we assign them. We do the tasks in there once per week. So actually, we create a new ticket with the state "pending" and the due date over a week and send the customer the new ticket-id. What happens now is, that every single mail that comes in for this ticket is actually setting the ticket state back to "new" or "open". But actually I want have it staying on "pending" until the real working day arrives. Any ideas? Greetings, Olaf