In Sent messages, I had Attachments, Subject, Recipient, Sender[@], Date, and Size set to display. Only Subject, Recipient, and Date were resizable. The resize double-arrow popped up for Attachments and Size, but it had no effect. It did not appear for Sender[@]. The Sender[@] column came from an addon (Show Address Only 0.1.9), which I initially thought might be a factor in its not being resizable. BUT... on further inspection (I originally said "playing around"), I find that if I move the Sender[@] column around, it becomes resizable. Any column moved to the rightmost position becomes un-resizable, but moving it away from the right end may make it resizable. Single-column columns (e.g., Thread, Starred, Attachments, Read) are never resizable, but the double-arrow resize cursor does pop up if you try to resize one of these (when not in the rightmost column). Sometimes when columns are moved, they are resized in the process. I guess I'm wondering if there's method to this madness.