Customers Cannot Receive Notification Emails

Customers Cannot Receive Notification Emails

We configured our Desk to send notification emails whenever a ticket is logged. But starting from yesterday(06/28/2021) (or could be earlier we just did not notice), our customers stopped receiving any notification emails.

We checked ZOHO Desk ticket history logs, as the snapshot shown below, notification rule was said to be applied,

BUT:

1. The customer did not receive the notification email, already checked spam/trash folders;
2. The "Sent" folder of our email account (we are using GMAIL) has no trace of this notification email at all.

This is an urgent issue for us, kindly please shed some light on this, Thanks!
(The ticket number of the snapshot below, is #939)