Starting on July 8th, 2019, administrators of GSuite accounts will need to explicitly trust 3rd-party apps that wish to access the Gmail API.
Trusting the Postbox application is a quick, two-step process:
1) Under Google Admin > Security > API Permissions > Gmail, click the link just under Gmail to review which apps have been using the Gmail API.
2) Find the Postbox entry, click the three horizontal dots on the right, and select "Trust."
