See the following page for info about 4.x and Cataclysm: http://my.curse.com/articles/speeddymon/848044.aspx
MailboxPlus is an addon designed to completely replace several separate addons with something Ace3-compatible, actively maintained, small, light, and fast.
-Dropdown list of mailed toons, which can be used to quickly send a mail without using the keyboard
-Take all items and money from the inbox -Take all items and money from a single mail -LDB support to track the number of mails pending and any Auction House invoices, including what sold and for how much -Communication with other users of this addon to let them know of an incoming mail, including the subject and any item/gold content
This project is in alpha status and it may have bugs. I also have a family that I take care of, so if I don't respond immediately, that is why, however I will try to fix errors that appear asap.
In the current release, only the dropdown on the send mail window, and some options exist (accessible thru the blizz options, or /mbp). Most of the options don't do anything yet, but the dropdown does function.
Note that the recipient list is sorted alphabetically, so any toon with a name starting in the later characters of the english alphabet, or any name starting non english characters will not appear if you send mail to a lot of recipients. I'm trying to think of a workaround, possibly letting you sort the list instead of it being alphabetical, or maybe adding a scrollbar instead of having to limit the length.
Note to localizers: Some of the strings are still in a flux, and may change. Please check back regularly to make sure strings you have localized still exist and are correct.