Backing up Gmail with Thunderbird to mbox, how?

[Nicholas]

I would expect this to be well documented and fairly straightforward.  
I'm using the plugin:

https://addons.thunderbird.net/en-us/thunderbird/addon/importexporttools-ng/

and just clicking a folder to export to an mbox file.  It's just not 
readable with mailutils.  Downloading a sample mbox works fine.  So that 
implies that the "mail" command is working at least well enough to read 
an arbitrary mbox file.  Perhaps this Thunderbird file is odd in some 
way?  That would seem unusual, as Thunderbird is quite mainstream, Gmail 
is very mainstream, and this is a widely used backup tool.  So that 
suggests that I'm not exporting "correctly" somehow.  But what 
specifically can be done differently is a mystery.

Has anyone had success, or failure, with backing up Gmail?  Did you use 
Thunderbird, or some other tool?  Any specific experience with this 
export tool?



see also:

https://askubuntu.com/q/1494163/847449




thanks,


Nick