I use the mailboxes and auction house fairly often, as do many many players.
Like many players I am constantly adjusting the position of each window (inventory window, mailbox (send letter), auction house (post panel)) to make dragging and dropping possible. This is also somewhat of a problem with the Trade screen. IN all above cases dragging and dropping items from inventory into the other UI panel is a mandatory action, and the UI has little to no functionality if dragging and dropping isn't possible. Especially with the auction house post screen it is diffucult to view both windows at the same time, and in fact impossible to complete the drag and drop function in most default positions. It is kind of ridiculous that the UIs main functionality is impossible in default position.
Neither the auction house, mailbox or tradescreen make much sense if you cant see your inventory at the same time, also it makes no sense that the position of the drag-to box for either UI is in opposite corners, making it nearly impossible for your inventory screen to be in the same position for using either. So if you set up for one, you block the other and vice versa.
I REALLY thinks the Devs should redesign the functionality of ANY drag and drop situation such that dragging and dropping is possible by default. sometimes the auction house window completely covers the inventory window, such that you cant even select it for dragging with your mouse.
Suggestion on how to make this work- In the auction house POST panel-make the characters inventory (just the actual pack displays) part of the Auction house UI, either to the right or left (probably left) so that you can drag and drop directly inside the same UI window. Do the same for trade and the mailbox (send letter).
There is a lot of coding for sure that would go into this but it is a DISPLAY only issue, no change in the game itself actually needs to be done, just display more information/functionality on 3 UI windows.
Players who also find this annoying, please post your agreement so the devs get an idea of how many of us would benefit from a change here.