We have found recently that if you have not removed the Received: headers
and other extraneous headers from the messages AND if you do not have
Notebook-Header= Full set in the list header, the Date: header must
come first in the message.  If Notebook-Header= Full then this constraint
does not apply.  I was still in the process of testing this thoroughly
before updating 8.10.3 of the site manager's manual and I have not been
able to complete my tests, but this does appear to solve the problem
if you can't easily rearrange the headers in your non-LISTSERV archives
when porting them.

Nathan