I managed to receive the bounced emails to my mailbox and I can now see the stated reason for th eissue, the bulk of the emails are marked as "expired" due to the following response:

"smtp; Message expired: unable to deliver in 840 minutes.< Our system has detected an unusual rate of unsolicited mail. To protect our users from spam, mail has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines."

The issue is exclusive to Gmail, no other email provider responded with a comparable message.

I followed their recommendation, but I am not sure how to reduce the rate on Postfix side. Can I do it per list? or once per server?

Thanks.