If in your statistics you are seeing a grey dot left of the message subject, that means that this message has been sent / delivered by us and received by the recipient server.
However, if the recipient claims that he has not received the message, the investigation must be conducted on his side (example: gmail or hotmail), as we have received a confirmation from their server that the message was accepted.
First, your subscriber should check in his spam folder if the email has landed there.
If this is still not the case, he should contact his email provider for more information.
If this is still not the case, he should contact his email provider for more information.