After creating a new Message, two headers are automatically placed into the message -- the first Date header is set to the creation date of the Message (essentially now), and serves as the "received" date if this is an inbound message, and the message ID.
Modify this Date header, however doesn't seem to be allowed.
Using setHeader(Header.DATE, "...") does not work -- the header isn't replaced. Instead, the second Date header (sent date) is generated and added to the headers. This is equivalent to calling setSentDate(...), but is, of course, not what I want.
Going about it in another way, with an iteration using removeHeader(), removes all existing headers except for the two originally created ones -- the message ID and the received Date.
So there seems to be no way to modify the received date (first Date) header after creating a new Message?