[MM3-users] Mailman messages are getting a TAG in some email clients.....how????

So I know about the setting "Anonymous list" but regardless of that setting some email clients put the list name in [] when it displays messages in your inbox. Does anyone know how email clients are doing this? I have users who email clients do not do this, and was wondering if it is a setting in their client or just not possible.

Here is a link to an image showing the tag for a list called TEST6 https://imgur.com/a/7VyJtsD