This does not really solve the problem when GM-->HM-->GM thread is concerned.
GM-->HM the header shows Message-ID as veiwed in Message. While replying from HM, the Message-ID of GM is replaced by something of HM as shown below:
Header of GM As Saved To Sent Folder: Message-Id: GMail.1077299057.175542330.65852969052@domain
While replying from HM the Message-ID is blanked Out but the GM recepient sees the following in the Message Header:
And there is nothing which can ever cause threading as there are no references to the GM Message-ID in above scenario. Any suggestions?
 It seems the complete threading is impossible with gm->hm mail thread because of hm removing all message-id, references etc to the mail being replied from the header. Is this a general phenomenon or something associated with my case only [/edit]