If your message is marked by our system as "Delivered", it means that we have received confirmation from the recipient server that the message has been accepted.
However, if the recipient of the e-mail claims not to have received the message, the investigation must be carried out on their side (example: gmail or hotmail), as we have received from their server a confirmation that the message has been accepted.
The recipient should first check their spam folder if the email made it there. If that's still not the case, they should contact their email provider for more information.