This update, to protect the games economy and make the gaming experience better, will now cause all mail containing gold, silver and copper coins attached to it to delayed for up to an hour. Just like it has been with mail containing items.

From the WoW Forums, written by Eyonix(Blizzard Poster):

As part of our continued efforts to protect the game’s economy and ultimately the game-play experience of each individual player, we’ve made a small change to how an aspect of our mail system works. Just like in-game mail transactions containing items, mail sent with gold, silver, and copper coins attached will in most cases now take one hour to reach the recipient. This change will help us to better ensure the legitimacy of such transactions and further prevent the unlawful influx of currency into each realm’s economy. Keep in mind that this delay will only affect the following mail transactions containing coins:

Regular mail with coins attached.
CoD payments received via the mail system.

Note: This will NOT affect mail sent from one character to another when under the same account.