Not really. You receive something that teaches you how to summon a dragon. Iirc most of the items you use to get mounts are something like "Reins of <mount name>". Also there's no real evidence that items in the mail appear in every mailbox. It could just be that every mailbox is just magically connected to the same place/dimension.
Regardless, both examples are obvious gameplay compromises that are not meant to represent the lore or the universe of Warcraft (which goes beyond WoW). The OP question is regarding a representation of the world and lore and is actually something that might be mentioned throughout quest text in BC or in description of the zones in the books or other sources.