While working on a complete migration of my previous mail accounts to GMail, (I am honestly one of the last Google Apps users that do not need to pay for the pro version…) I stumbled upon „ImapSync“ (which I refused to use because of the price) and later on, upon „ImapSize“. While ImapSize was able to download all my mails to the handy .mbox format, I wasn´t able to upload my mails to GMail, because of the following error:

„Can´t continue uploading messages: SEARCH completed (Success)“

After testing around and trying to just Copy and Paste the bunch (>20.000) of mails from the old to the new account just by using Outlook, I recognized, that this will not work at all. Outlook just crashed after a couple of minutes. To be clear, this solution will of course work, if you only have a couple of mails to transfer, but I was searching for a general approach with thousands of mails and inboxes sized > 3GB.

So, here is my solution for this problem:

1. Download a new version of ImapSize.
2. Set Up your old Mail account in ImapSize and press „Crtl“ + „S“ to get the size of your IMAP folders.
3. Right click on each of the folder you like to migrate, choosing „Download Messages“, followed by „To mbox (Unix mailbox)“.
4. Download and install Thunderbird.
5. Set-Up your new (GMail) account in Thunderbird (you can use the auto detection for the servers, based on a well-working ISP look up)
6. Open the „Add-ons-Manager“ in Thunderbird and download the „ImportExportTools“. For my migration I used version 2.8.0.1. which was working quite fine.
7. Add a new local folder, right click and choose from the new submenu „Import / Export“ the option „Import MBox file“ for each of your saved mail folders
8. Just select all mails in the particular local folders and the  drag and drop all mails to the IMAP folders. Thunderbird will now spent a lot of time (depending of course on your Internet connection and the amount of mails), synchronizing your mails.