Separate windows for separate Apple Mail accounts

I have multiple mail accounts and several of them have many sub mailboxes (some on the computer and some in the email account). I do voluntary work for a small charity and so I want to easily use that mail account to manage expenses (also with a few other users). Therefore I have set up an expenses email account for the charity where users will send their expenses and then when one of us processes the expenses they can forward the email to an authorisation group and file the expenses email to a sub-mailbox called actioned. The trouble for me is that, on my MacBook, although I can see the inboxes at the top of my email system, there are so many sub mailboxes for my other mail accounts that I have a massive scrolling task to find the actioned mailbox to drag the email to. The iPhone has a folder action which is great. Ideally I could open each mail account in a different window on my MacBook and then it would be easy.