I have a client you needed a good 4 years of emails imported into GMail.
At first I thought, okay there must be a nice way of doing this. Then I found some python scripts which did some magic, and I thought .. ahh it works. And then I found without testing or checking, that perhaps they were not what i was after.
I needed the following to occur
(a) the emails appeared as if it was created when it was
(b) it showed it was sent by or received from as per the original
(c) there was no alteration to subjects or anything etc.
So I managed to try a "rule" in Mac mail to forward all mail under certain criteria to the gmail address, and then apply the rule to the mails in question. But this was a bit dicky. It added the Fwd: subject prefix and amm mail appears as "me".
I then thought about it ad it was simple...
Just attach the Gmail mailbox in question as an IMAP folder in mac mail.
Import (or no need if already there) the old email from wherever to mac mail. Then .. hat kiddies ? .. drag and drop the mail from the various folders to the IMAP folders.
Simple. A quick check and the mail appears as if sent in 2007, from whom, to whom. (this was the sent mail) .
Nice. No scripts or crap.. just time waiting to MacMail to do it's thing. I suspect my client will need to expand their data limit with Google.. but hey, what a service!