Greylisting is another measure in place to protect you against spam.
It works by temporarily rejecting email from email addresses it does not recognize. If the email is from a legitimate email server it will be queued again for delivery and arrive a short time later. This generally does not happen with spammers and if they do retry the delay often allows our spam filtering programs to update to block their current spam content.
This means you may experience a delay in receiving emails from new contacts that have not got their email set up correctly.