A Please set "Display Message Thread" ON in "Settings" page.

N.B.
To tell the feature, thread function can put emails together as a same conversation, but not by mailing group. Because ibisMail doesn't include function to send/receive by mailing group.

Furthermore, ibisMail doesn't include option to show incoming/outgoing emails together unfortunately. Incoming emails can be put together as a same conversation, but thread function sort outgoing emails toward "Sent" box.