Repair Headers with Envelope Information via IMAP

[Henning]

Hi there,

I have a FirstClass E-Mail Server which has unfortunately a broken IMAP-Module.
With some effords at least I'm able to fetch messages from the Server with
offlineimap to migrate a couple of users to another server.

Received messages are fetched correctly, but Sent messages break because the
FC-Server stores the Subject, Date and Sender not as Header-Line but only in the
ENVELOPE-Line.

I want now to repair these Headers with the corresponding fields from the
Envelope with offlineimap (the only program i got to work with this server). But
I think I have to modify the code of offlineimap itself, but I don't have a clue
where to start. Could someone help me on this way?

Kind regards,
Henning