If the emails are residing with GCE. Do kindly check and ensure that the Email routing in the cPanel (GCE) is set to Local Mail Exchanger.
If the issue persists, do drop us an email via [email protected] with the following details so that we can further assist.
1) Sender email address
2) Recipient email address
3) Approximate Date and Time of the email sent
4) Subject of the email