[OT] E-mail threads do not display correctly in my mail client

[Jerry]

Why do e-mail threads from this list not display as properly threaded messages 
in my e-mail client?* I'm on many lists and this is the only one with this 
behavior. I'm using Mail.app on OS X.

* Related messages all display as separate items instead of being displayed as 
one item with a disclosure rectangle. This makes reading and mass deletion 
harder than necessary.

Jerry