The email goes on to encourage preorders of Warlords of Draenor.
The Iron Horde is coming and to help prepare you for the invasion, we've given your account, [account name], a FREE digital copy of Mists of Pandaria®. When you reactivate your WoW subscription, you'll be able to battle to level 90 and get ready for the launch of Warlords of Draenor™ on 11.13.14.
It's currently unknown whether all lapsed accounts are receiving this offer or not, but we've seen multiple reports of it over the weekend -- and I've received one myself for a secondary account I haven't played in a few years. Even if you haven't received this email but are considering a comeback, it may be worth checking your account page. Your lapsed account may have already been upgraded for you.
This coincides with a general price reduction for Mists of Pandaria. The expansion is now listed at $9.99 on the Blizzard store.