How to combine multiple mailboxes in Mail.app? My OS is Mavericks.  The mailboxes shown are grouped under headings MAILBOXES, SMART MAILBOXES, ON MY MAC, and MY EMAIL ADDRESS.  I have Sent and Trash under each.  Their contents are not entirely duplicative.  When I go to mailbox/use this mailbox as, the options are grayed out.  In Preferences/Accounts/Mailbox Behaviors all options are checked to 'store messages on server'.  How do I combine these mailboxes?
Thanks,
Rick
In the Menu Bar, go to Mailbox after selecting the sub-mailbox, one of your Sent folders, for instance, and then choose "Use This Mailbox For…". You can lump the redundancies together this way, but it's true that you will not be able to completely customize all the boxes.