Google Postmaster:

Useful to pull in delivery data for marketing teams who send to b2c audiences and tech teams monitoring authentication etc.

The Postmaster Tool API allows you to gather metrics on bulk emails sent to Gmail users and import data into, or merge that data with, other systems. Metrics include the percentage of your email marked as spam by the user and delivery errors. For a full explanation of the metrics provided by the Postmaster Tool API, refer to Dashboards section of the Postmaster Tools help center article.

https://developers.google.com/gmail/postmaster