When migrating, it’s so easy to lose old stuff; be it moving houses, buying a new computer (or reinstalling the old) or upgrading the mailsystem to something new.
Sometimes this might be the purpose, getting rid of old junk and starting fresh on the new location.
I must admit that I am a collector; my c64 history, all the articles I ever wrote and ALL mails ever written.
For quite some time, but many years ago, did I use Forté Agent. It was brilliant as it could handle mail and Usenet news in one package. I was even a paying user for very long, provided points of view for the future development even if I now realise that there was a lot going on behind the scene with the program. It was sold to Alcatel and then divested again from Alcatel (who naturally failed big doing anything relevant to the product!). Eventually Mozilla Thunder showed up – it was just as competent, and it was free. News was deleted from the offerings of my ISP and the other News servers that could be accessed for free were closed. With News not being accessible, migrating to Thunderbird I found that I had produced quite a lot that was now locked in, in the Agent proprietary storage format. BUMMER!
I have migrated the archive with every swap of computer since, both for me and my wife until today where the ultimate solution has been applied – it’s now being uploaded to GMail, so it’s not available and not bound to me remembering to migrate it every time. RELIEF!
So how did I do it?
1) First step – extract it from the Agent cage.
This is a tricky step. The only relevant solution is to extract it using the only relevant converter there is. Aid4Mail. This program converts also very large mail boxes between formats. I used Agent as source and Outlook PST as destination.
2) Second step – get it to GMail.
The trick here is to use Outlook as the vehicle that takes you there. Basically I followed this guide.
What you need to do is to
a) Enable IMAP in GMail. Under Settings, Forwarding and POP/IMAP there is an option to simply enable it. Save the setting and that piece is taken care of.
b) Define a GMail account via IMAP in Outlook. To be honest, this is the tricky part. Defining username, password and servers might be straight forward, but you need to do the advanced setting, defining SSL and also a specific port. There are guides available for this, for example this one (Swedish I’m afraid but I’m sure you can find it in your language as well or have it translated).
c) The guide referenced on the first line of this section suggests you import messages from the PST file. That’s not really needed. It’s even a fairly clumsy solution. All you need to do is to open the PST file. (Select Open from the Archive menu). Quite a lot easier as the target is not to have the mail available in Outlook when I’m done with this. If it was, you can of course also copy the PST to the hidden directory where all of the others are also placed.
d) Now the really easy part – just select the directory with the mail and copy it got the GMail folder. It will naturally take LONG to copy data over IMAP. We are talking hours even with modest volumes, but who cares – let it work overnight and wake up a happier person!!! 
Learnings
Did the above sound easy? In theory YES, but in practice NOT AT ALL!
My transfers were interrupted many times for various reasons.
Key one was that it wasn’t possible to create subfolders. Thing is – I didn’t have any subfolders so this was in fact an error that indicated unsupported characters in the directory names. Of course the “/” is not to be used, but I also guess that the “!” and the quotation mark itself.
Secondly there is a restriction on length of the tag name. If the foldername that gets to the tag is longer than the allowed tagname, then you’re our of luck.
