OkiOki itself sends your invoices to your customers via the email address firstname.lastname@example.org. This means that you will not find your invoices in your own e-mails.
That is why you will find the 'OkiOki status' of your document on every document detail at the bottom of the right-hand info bar, including whether and to whom and when your invoice was sent.
If OkiOki is unable to deliver your invoice (because the recipient's email address no longer exists or there is an error in the email address), it will notify you by email. At the same place in your document detail you will find an error message with the option to change the e-mail address and resend the e-mail.