As some of you may know, there is a 1 minute cooldown on mailbox useage each time a new item is sent to your mailbox. If you buy item A at the auction house, then take it out of the mailbox, then buy item B, you have to wait until the minute is up before it shows up in the mailbox. This is a minor inconvienience, but can be terribly annoying especially for inscriptors like me. I will often cancel 1000+ glyphs at once, and have to wait 1 minute in between every 50 glyphs to take out more.
Anyways, the solution is simple. Simply log out and back in, and you'll instantly be able to take the stuff out of your mailbox.