offlineimap VS apt-offline

There are also isync and OfflineIMAP which sync email locally.

I'd consider using the Bridge + offlineimap (or a similar tool). That can connect via IMAP to both the Bridge IMAP and your destination IMAP service and copy all mails automatically that way.

As u/alex_herrero says, Proton Mail Bridge is very much useful. I'm using this myself together with offlineimap to have a local backup. I also use the Bridge with Thunderbird too, which also works quite well (especially the v3 beta).

For the sync / import functionality the MSP uses offlineimap, which is another counterpart to IMAPSYNC. Changing the import tool will not bring up the solution to the main problem, that I am not able to reset the imported stuff back to zero in order to test the different scenarios of the main use case.

Don't risk relying on someone else's hardware or service, use something like Offlineimap or Mbsync to sync your emails to your computer then automate multiple backups.

Many people use OfflineIMAP.

I've used offlineimap in the past: http://www.offlineimap.org/

I'm looking into replacing Thunderbird with mu4e (for the first time, despite being an Emacs guy for 15 years) and I got unexpectedly stuck at step 1, dowloading my email. offlineimap is broken on newer Pythons, getmail doesn't support Python 3 at all, and fetchmail is infamously inscrutable. It all seems a bit ridiculous, but I guess it speaks to how webmail has come to dominate proper email clients. Any recommendations?

Neither a mutt, nor an offlineimap user, but I believe those two are often used in combination: http://www.offlineimap.org/

If you have a USB stick and access to a Windows computer (neighbours, friends, etc.) with Internet access, you can use https://github.com/rickysarraf/apt-offline for this.

Take a look at https://github.com/rickysarraf/apt-offline

One possibility would be http://rickysarraf.github.io/apt-offline/. With this tool you can copy the necessary packages, including their dependencies, to a USB stick and then install them on a computer without internet access.

There was apt-offline that did a very good job as far as that goes.

Check out apt-offline which is packaged for Ubuntu.

I have also seen apt-offline.

Here's the discussion about the "ERROR: Path for keyring invalid" problem with apt-offline: https://github.com/rickysarraf/apt-offline/issues/102

If you cannot establish another network connection, you could solve the problem by using https://github.com/rickysarraf/apt-offline. You will need a Debian based distribution with internet access (a live distribution should probably work) and an USB Stick. This will copy the required package including dependencies to a USB stick. With this you can install the package on the computer without internet connection.