In landscape mode tap the Mailbox button at the top left of the email list. Keep tapping it (if necessary) until you see a list of Inboxes and Accounts. Look for an Outbox. This is a temporary folder that only shows up when there is mail waiting to be sent.
Keynote files can be very big, and take a long time to be sent. If the email is sitting there with no indication that it is being sent (you should see the usual pinwheel activity icon next to it), then try tapping the circle/arrow icon at the bottom left of the email list.