Migrating to a new Gmail email account is a lot easier using Google Takeout this Thurderbird Add-on ImportExportTools NG. I have been splitting up my email accounts so that I have several distinct accounts - more on that below. The gist was that in the past I was forwarding all mail into one account which I'm no longer doing but now I want to move all that old mail, ~30,000 emails, into a different mailbox. Recommendations online were to connect Thunderbird to the two accounts and move messages in batches but in practice Gmail times out these connections very quickly and the batches are way too small. Instead I ended up exporting my email and importing it - why Google does not offer an import of the format in which they export boggles the mind, and while it's possible I've simply missed proper screen where this can be done, somehow I think not. This seems to be one of those things that is easy enough, but not obvious and took a bit of searching.
The easiest process I found was to use the ImportExportTools Add-on:
- Spring cleaning time of your old email! Newsletters and automated notifications that were being filtered to a folder go 💥, gone.
- Go to Google Takeout and export mail. This produces a ZIP file which you can extract and there is a single large MBOX format file
- Install Thurderbird and the ImportExportTools NG add-on
- Import the MBOX as a local folder with ImportExportTools