This is due to so-called greylisting. In such cases, emails from an unknown sender are rejected by your e-mail provider for security reasons.  

The intention is to avoid SPAM e-mails, as senders usually do not try to deliver the email again.  

For a new sender, the delivery will be significantly delayed or the email will not be delivered at all. This also applies for emails with a verification code. 

You can find more information on greylisting here.