#4 - March 29, 2010, 12:08 a.m.
From what I can see, Violete, your account isn't upgraded to Wrath of the Lich King. You are currently on a Vanilla World of Warcraft trial account.
Because it is a trial account there are some restrictions in place. One of those restrictions is that trial accounts are unable to retrieve or send mail. Your friend can send mail to you but you will be unable to retrieve it until you upgrade your account to a standard account.
http://us.blizzard.com/support/article.xml?locale=en_US&articleId=20590 Now, you can upgrade one of two ways. You can purchase an online upgrade through the account management page or you can purchase a retail copy of the game from a store.
If you upgrade online the majority of the trial restrictions may remain on the account for up to 72 hours while the purchase fully processes. If you purchase a physical copy of the game though all trial restrictions will lift immediately.
If you decide to pick up a physical copy of the game remember to log into your current Battle.net account and upgrade your current trial account. Do not create a new account otherwise you won't have access to your current characters.
https://us.battle.net/account/management/add-game.xml? It should also be noted that you can purchase the Battlechest version of the game, this comes with both the Standard Vanilla version of the game as well as the Burning Crusade expansion for a reduced price.
I hope that helps, Violete.