Select newly inserted messages (FUMLUB)
In [Options/Prefs window] > Extras ...
If you select this option (and you’re sorting your mailbox index by ascending date), the first message of the most current block of unread messages is highlighted in the table of contents window whenever a mailbox is opened. (For mailboxes that are already open, the highlight position will not change.) FUMLUB means First Unread Message of the Last Unread Block. For example, you may have a series of unread messages in a mailbox scattered throughout your table of contents. The first message of the last block of unread messages listed in your table of contents is selected.
If the date sort is descending, then the feature becomes LUMFUB (Last Unread Message First Unread Block).
FUMLUB (or LUMFUB) only works with the following index sorts: Date, Order Received, and None. If any other sort is selected, the message selection feature is disabled.