[mailman] some mails do not arrive

Hello, I work on a website of an association and we manage the lists with mailman ( 80 people) on a cpanel (o2switch). Some accounts (at least two) are not able to post: they don't get error messages, the mails are archived, but don't reach the list subscribers. All email addresses are configured in the same way on maiman. Do you have a lead? (I'm not a developer, just a web integrator) Thank you