The best way to verify that autoresponders/notifications are being received is to try at various domains. So if someone at Gmail is not receiving the expected email, try Yahoo or if someone in your organization is not receiving the expected email, try Hotmail, etc.

If it is a work email, most likely Luminate's outbound domain and IP addresses are being blocked and will need to be whitelisted by their work's IT department

Please keep in mind that we cannot guarantee 100% reception of emails sent out. Once the message leaves our system, it will be relayed between mail servers not under our control, and subject to many potential mishaps. We cannot control all of the computers involved in the delivery, although we make every effort to adhere to reasonable spam and email construction standards. If one of the mail servers or the end recipient's computer decides to delete or not deliver the message, there is not much that can be done about that. If you receive consistent and frequent reports of this type, then please contact support to see if there may be something unusual going on that can be fixed.

They may also be found in Gmail's "Promotions" tab as seen here.

How do I generate a receipt for a constituent that did not receive an autoresponder?