If I'm off-line, and I send an e-mail, it stays as "unsent message".
Is there a way to cancel or delete or edit those unsent messages? (using the default e-mail app)
Go back as far as you can in the Mail view. There should be an Outbox folder.
The unsent mail will be in there.
Then you can use the "Swipe to delete method" or tap "Edit" button and delete away.