You can increase the speed your mailbox loots items by downloading an Addon called Postal and changing the speed it loots items from the mailbox. Here is a step by step guide on how to do it!
1) Download Postal Postal - Addons - Curse
2) You can either edit the value ingame or go into your WoW folder > Interface > Addons > Postal > Postal.lua and find the part that says OpenSpeed = 0.50, change it to OpenSpeed = 0.20,
3) Save your changes and re-load your UI.
4) Now that your mailbox is looting fast, lets get rid of the delay inbetween every 50 posts! You can simply type /rl and it will reload your UI for you. Once your UI is reloaded you just click the mailbox and you can loot all your gold/items again without having to wait!
Happy Looting !