The error was reported as

Auctionator\AuctionatorHints.lua:1177: Usage: GetInboxItem(messageIndex, attachIndex)
[C]: in function GetInboxItem' Auctionator\AuctionatorHints.lua:1177: in function <Auctionator\AuctionatorHints.lua:1176> [C]: in functionSetInboxItem'
FrameXML\MailFrame.lua:300: in function `InboxFrameItem_OnEnter'
Postal\Modules\Express.lua:42: in function <Postal\Modules\Express.lua:41>
(tail call): ?
[string "​_:OnEnter"]:1: in function <[string "_​:OnEnter"]:1>

Locals:
(_temporary) = 6
(_temporary) = nil

There may be more but for now these are the changes needed to update to legion

In AuctionatorHints.lua

Line 1177

Change

local _, _, num = GetInboxItem(index, attachIndex);

to

local _, _, _, num = GetInboxItem(index, attachIndex);

Line 1184

Change

local name, _, num = GetSendMailItem(id)

to

local name, _, _, num = GetSendMailItem(id)

Fixed in release F