Hello,

You could also download all mails directly from Gmail, through a tool called Google Takeout. Here's Google's own support page on how to do that.

Thanks Gert. I already did that - 20gig of data on an external hard drive in an mbox. But, I couldn't get it to re-open in Thunderbird even after downloading the import-export plugin. In the meantime, I've got thunderbird to fully download, directly from google's cloud, all my emails, and need to export from thunderbird to the external hard drive. I'd like to delete all these emails from my computer to save hard disk space.

If there is a hard way to do things, you apparently are doing it.

You have your MBOX download. Now use an MBOX to EML converter to convert it to EML files and forget Thunderbird entirely. There appears to be a school of though that Thunderbird is good for this Google backup rot. It is not. It can be achieved only with attention to detail and usually a lot of grey hairs.

Typing MBOX to EML converter in Google gave be a quarter of a million hits. Most of them will do the job, if they can cope with the 20GB file size. Why? because it is simply slicing a mail off the top and saving it as an EML file. The only difference between EML and MBOx is MBOX has lots of mails one following the other and EML has just one, all in text format. I would surmise you could write a batch file to do the job, but let someone else do the coding if at all possible is my motto...