Some e-mails, not spam, are invisible in the iPad. They are just not in any folder, yet, they are there. They show up on the iPhone, but not the iPad. I checked by logging on to a webmail account, and they were there. Very odd, eh.
This is the Apple email client. It would make some sense if it appeared in trash, if it was misdirected, but it's not there. It's not an old message, it's a new one, so it should appear with the other recent messages.
I was going to suggest deleting the app and re-loading it, but you cannot do that yet with this Apple app. Try this: Choose "take all accounts offline", and wait 10 seconds. Choose "take all accounts online".