Patch 5.0.4 - big changes in advance of MOP

[Kevin]

I think it's been that way for awhile - when the wife and I were levelling our DK/Shammy combo I had to mail the plate heirlooms to her because I couldn't trade them to her.

I guess in the Blizzard world of coding, the mechanics of transferring an item via trade window is that much different than that done by a mailbox.

Trades are "live" transactions, mailboxes are "queued" transactions. Trade windows are also an easier target for hack/dupe exploits than the mailbox interface because you can have control of both ends of the transaction if you are trying to cheat around the system (and there were some successful gold dupes in the early days). So yes I could see there being differences in how those are coded and what attributes the item objects have and calling a completely different method to handle the transaction.