OfflineIMAP is a GPLv2 software to dispose your mailbox(es) as a local Maildir(s). What it allows you to do is to make a complete sync of your email over the IMAP protocol and in essence keep local copies of email. This is a GUI-less alternative for a thunderbird email backups, plus you also forgo the usablity of a proper email client which would allow you to make replies to old emails. For these reasons, I personally recommend using thuderbird configured to keep deleted copies of emails as an alternative, but some needs are more basic, and this will get the work done.

Installation

OfflineIMAP comes in package repos for all major distos, so installation is as easy as running

$ apt install offlineimap

Or whatever is your package manager of choice.

Usage

Create a file ~/.offlineimaprc and fill it with the following content

[general]
accounts = <Any Name>

[Account <Any Name (Same as one above)>]
localrepository = Local
remoterepository = Remote

[Repository Local]
type = Maildir
localfolders = <Path to the folder for backup>

[Repository Remote]
type = IMAP
remotehost = <host>
remoteuser = <user>
sslcacertfile = /etc/ssl/certs/ca-certificates.crt

Then all you need to do is run

$ offlineimap

It should prompt for your login password. Enter that, and you should see the emails being downloaded locally and put into the folder you provided.

Restoring Emails

Flipping the localrepository and remoterepository should get the job done. It has not yet been tested.