You know the headings on the mail screen? Date, From, Subject, etc.? You see the little triangle buttons next to those headings? If you click on the heading, it sorts by that field. If you click on the triangle button, it reverses the sort order. Unfortunately, it doesn't save those settings.
If you want to make these changes permanent, edit /home/httpd/html/horde/imp/config/mailbox.php3. Find the section that says:
/* sort order. can be one of:
* SORTARRIVAL
* SORTDATE
* SORTFROM
* SORTSUBJECT
* SORTSIZE
*/
$default->sort_crit = SORTARRIVAL;
$default->sort_dir = 0; // 0 = regular direction, 1 = last/highest first
It should be obvious what to do here. This file doesn't seem to be generated from a template, so you should be able to directly edit it.