I once heard a talk by the gmail team. They pointed out that some of the Ops people at Google had simply immense amounts of email.

An anecdote - they got an email from someone who said, "One of my mailboxes takes five minutes to open," and the speaker said, "But we already knew this guy's email address really well, because that mailbox had over several million messages in it and was one of the largest in the whole system."

So this guy was the Sean Kernan of Google.