A constituent took an action (e.g. made a gift) and did not receive the autoresponder and/or the admin didn't get the expected notification. Can you confirm this was sent? All of the SPAM filters have been checked and the email was never received. Could you please confirm that the messages are being sent as they should be?
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.
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.