Tyren has just posted on the Blizzard forums that they found all the missing mail and can recover it and get it delivered:

Tyren:
Please keep an eye out for a lot more mail to start showing up in your mailbox in the coming weeks. We don’t have an ETA on when the process will be complete, but we will get everyone’s expired mail back to them as soon as possible. However, please note that this process will take a few weeks to complete.

Read the original post here (http://forums.worldofwarcraft.com/thread.aspx?fn=wow-general&t=2240252&p=1&tmp=1#post2240252).