This was the response I had received from Hostmonster.
Hostmonster - The emails sent via Mailman use the IP address 127.0.0.1, which is the default IP address of the server. Unfortunately, when emails from this address are dropped due to mailing list restrictions, it becomes challenging to lift these restrictions, thereby preventing us from resolving the Mailman-related issues effectively.
Currently when the emails are sent from mailing lists, then those emails will be marked as spam, even though the user didn’t send large volume of emails from the server.
We have observed similar issues from other customers using MailMan for bulk emailing. I am afraid, there is currently no workaround for this issue when using MailMan. Given these challenges, I kindly request you to consider an alternative solution, such as phpList over Mailman, for bulk email management.