Despite its issues, Imapsize might be worth keeping around installed, say, in a virtual machine running Windows XP for diagnostic and maintenance purposes for IMAP accounts. The software is, as far as I know, unique in its set of features. Imapsize gets a rating of 2 out of 5.
So, having given up on Imapsize for backup purposes, I did a little more research and discovered a program called, rather unsurprisingly, Gmail Backup.
Created by Jan Švec and Filip Jurčíček, Gmail Backup is free and, unlike Imapsize, seems to be bug free.
To execute Gmail Backup you only have to enter your Gmail account details and off it goes ... downloading all of your messages onto your local file system in Windows Mail E-Mail Message (.eml) format.
The download is into files named "YYYYMMDD-hhmmss-nn.eml" -- only the "nn" part needs explaining; contrary to the online documentation, this is a string formed by appending the beginning of the subject line to the sender's email address followed by a sequence number to eliminate naming collisions.
These files are saved into a subdirectory hierarchy of year/month/day under whichever target subdirectory you select.
In my testing on an asymmetric DSL connection (3Mbps down, 500Kbps up) I saw an average download rate of roughly 55KB per second which equates to about five hours per gigabyte.
You can select a date range to download and optionally restrict downloads to "Newest emails only" which performs incremental backups of your Gmail account (Gmail Backup simply saves a timestamp of the last backup in a control file in the backup subdirectory).
Gmail Backup also supports command line arguments such as:
gmail-backup.exe backup_dir email@example.com password 20070621 20080101
... and appending "-stamp" will perform an incremental backup.
Gmail Backup (which gets a rating of 5 out of 5) has solved my worries about a "Googlesplosion" so that I can now concentrate on my Christmas cooking plans and it is proof that a good recipe can solve any problem.
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