External domains are trying to send emails, but they are never delivered.
- Check if the email was delivered to Google.
- In the Admin console, go to Menu > Reporting > Email Log Search.
- Next to Date, select a date range from the list.
- In the search fields, enter any information you have and click Search.
- Under Subject (or Message ID), click any of the subjects to see message details and delivery status.
- If your missing message is listed but you still can’t find it, try these common fixes for missing messages.
- If you don’t see anything about the message, it wasn’t delivered to Google’s servers. In this case, contact the message sender.
Not able to determine the cause, as ELS doesn't show the emails, the issue is on the sender's side.