The Web GUI allows sorting by size, subject and date, but not by other columns.

Second question:

The manual lists the following columns; is this the COMPLETE list of available columns?

SOGoMailListViewColumnsOrder
(not currently editable in Web interface)
Parameter used to specify the default order of the
columns from the SOGo webmail interface. The
parameter is an array, for example :
SOGoMailListViewColumnsOrder =
(Flagged, Attachment, Priority, From,
Subject, Unread, Date, Size);

Which columns can be used for sorting, is it possible to chain sorting,
e.g. sort by Priority first and by Date second?