Mailman auto-unsubscribe by bounce while all mail ids are valid in the list

I am working on Mailman version 2.1.12-14.el6.x86_64 and sendmail that came inbuilt with RHEL 6.0. My mailman does not directly send mails to the destined domains. It relays to another appliance called "Ironport" and that in turn verifies various parameters like spam, virus and so and then sends to the destined domains. But, I recently observed an issue on this setup after lot many years of usage, suddenly from few days, the maximum no of users from one of the mailing lists got auto-unsubscribed with reason "BY BOUNCE". I tried searching in mailman logs and many websites for the solution but nothing helped. The only thing I could get from previous "subscribe" log is : /var/log/mailman/subscribe-20181028:Oct 27 09:01:05 2018 (17828) : deleted ; disabled address /var/log/mailman/subscribe-20181028:Oct 27 09:01:05 2018 (17828) : auto-unsubscribed [reason: BYBOUNCE] The above message is an example. Almost 142 mail ids got deleted with the above reason. My query here is, if mailman is not sending directly the mails to the users, what is causing mailman to bounce back to the sender that leads to the auto-unsubscribe from the list.