Many people found that they did not have the mail that was previously stored in their mailbox prior to the reset.
This is, of course, a big deal for bank alts and others who send tens of thousands of gold through the system. Blizzard has shut down the realms again, and is aware of the problem.
Update at 3:35pm: Syndri tells us that they are actively looking for a fix, and that as of yet none has been found. See our realm status post for more information about the extended downtime.
Update at 10:34pm: Mail will be reimbursed. More information.
We here at WoW Insider would like to encourage Blizzard to just fix it!
Stay tuned for more updates.
Updated 2:32pm: Syndri commented: ""...we are able to view many of the missing mail articles within players' inventories. Though this is promising, we've still neither ETA nor confirmed resolution available."
Updated 3:11pm: Syndri now comments: "This issue remains a priority for our technicians, database administrators, and in-game staff at this time. We've no ETA available, though work is hard underway. As a result of these efforts, we are continuing maintenance on all realms. Though we are loathe to limit your available game time, we also want to ensure that this issue is addressed to the fullest extent possible. Thank you all for your continued feedback and responses. As more information becomes available, I will provide it herein."
Are you ready for the challenges that await you in Northrend? WoW Insider has you covered with our 8-day guide to Preparing for Wrath. Also, check out our feature on AddOns new, old and outdated and Essential AddOns for Wrath written especially with the expansion in mind.