Where do i put an outgoing envelope if my mailbox is shared by other houses in the neighborhood?

I live in a neighborhood where the mailbox has alot of slots and each one needs a key for it. Is there a slot where i can put outgoing mail?
I think you are describing a MBU which is a multiple box unit. The unit has several boxes with seperate doors, each door is opened by a seperate key for each household. The carrier opens the back of the unit to deliver mail to each household. There should be one box that has an outside slot for you to deposit your outgoing mail. The carrier gathers the mail out of that box when they are depositing mail into the seperate boxes from the back of the unit. If there isn