Gmail’s content can be retrieved via IMAP, and we’ll use this way to backup all our mails thanks to OfflineImap, a generic IMAP synchronization utility.

Let’s start by creating a dedicated configuration file in your home directory. Its content is quite straight-forward, as you can see in my /home/kevin/.offlineimaprc, which backup two Gmail accounts:

[general]
accounts = gmail_account1, gmail_account2
maxsyncaccounts = 3
ui = Noninteractive.Basic

[Account gmail_account1]
localrepository = gmail_account1_local
remoterepository = gmail_account1_remote

[Repository gmail_account1_local]
type = Maildir
localfolders = ~/gmail-backup-account1

[Repository gmail_account1_remote]
type = IMAP
remotehost = imap.gmail.com
remoteport = 993
remoteuser = account1@gmail.com
remotepass = XXXXXXXX
ssl = yes
maxconnections = 1
realdelete = no
folderfilter = lambda foldername: foldername not in ['[Gmail]/%s' % f for f in ['All Mail', 'Trash', 'Spam', 'Starred', 'Important']]

[Account gmail_account2]
localrepository = gmail_account2_local
remoterepository = gmail_account2_remote

[Repository gmail_account2_local]
type = Maildir
localfolders = ~/gmail-backup-account2

[Repository gmail_account2_remote]
type = IMAP
remotehost = imap.gmail.com
remoteport = 993
remoteuser = account2@gmail.com
remotepass = XXXXXXXX
ssl = yes
maxconnections = 1
realdelete = no
folderfilter = lambda foldername: foldername not in ['[Gmail]/%s' % f for f in ['All Mail', 'Trash', 'Spam', 'Starred', 'Important']]

Notice how we use a Python lambda expressions to filter out some Gmail’s virtual folders.

Then all you have to do is to launch the offlineimap command-line itself with the right user, for example in a cron job:

00 20 * * * kevin offlineimap

A final warning: OfflineImap is fully bi-directional. This mean local deletion propagates to the remote server. This is can be quite dangerous so be careful not touching your local folders. If for any reason you’d like to reset your backups, stop OfflineImap processes first, then remove its cache folder (~/.offlineimap/) before removing the local folders themselves (~/gmail-backup-account*).

Also, intensively playing with OfflineImap to adjust its configuration may trigger the infamous Gmail’s “Temporary Error 500″. In this case don’t panic: it seems to be a common Gmail’s auto-immune response against suspect activity. It happened to me and in the end my account and mails were safe: I just had to wait a few hours to let it resume normal operations.