[horde] IMP sort by date

[Chris St. Pierre]

One of my users has a few messages in her inbox that do not get sorted
properly by date.  The messages are old, all received in 2002,
although one has a bogus date header from 1904 (!).  Yet all of them
sort as though they were received today.  This doesn't happen in other
MUAs.

What could be causing this strange sort?  What header field (or
combination thereof) does IMP consider when sorting by date?

Chris St. Pierre
Unix Systems Administrator
Nebraska Wesleyan University