Mail: mystery icon in mailbox column headers

[doctormelodious]

For no apparent reason, now when I create a new mailbox, the first column header contains a mystery icon consisting of two arrows, one pointing northwest and the other pointing southeast.

When I right-click on the headers, there is no corresponding item in the pop-up menu (e.g. Attachments, To, From), so I have no clue what it is, nor can I hide it.

Mailboxes I created prior to whatever it is that changed do not have this icon. I have not changed my OS, nor the version of Mail I am using. Can anyone tell me what this icon/column is for, and how I can get rid of it??

Thanks!
DM

 

[DeltaMac]

That's Mail's icon for Threads (sorting messages with common subjects, etc.)
Turn that off from the View menu, and click on Organize by Thread (which should both unselect it, and remove that double-arrow column)

 

[doctormelodious]

D'oh! Somehow I had checked that in the View menu, and all new boxes were inheriting that setting. Thanks for solving the mystery!