I'm updating this column (for more than 48 hours now) on a RAID5 server.
RAID5? That's probably the worst performing RAID configuration you can have and is usually advised against on this list. You would be better off with RAID 10, RAID 1+0 or even RAID 1, but you would be using more disk space.
That said, if your IO is not being saturated by that query, you could split the update across multiple CPU's by dividing up the email_id's over multiple queries that you run from a session each.
-- If you can't see the forest for the trees, Cut the trees and you'll see there is no forest.
