There are new mail rules released in the latest patch that have been confirmed by Eyonix on the general forums.
All mail from characters on the same account will now appear instantly in their mailbox. This includes all items and coin so no more having somebody hold items for you while you switch toons.
All mail sent to any other account including both Items and coin will take one hour to reach the recipient. This is a measure to combat gold selling and to make the process a little bit more inconvenient for both sellers and buyers.