I'd like to backup/archive my Sonic mailboxes to my local drive. Is this possible?
Use any mail program application that uses POP (or another service Sonic supports). I just use Apple Mail. Once set up, it does it automatically every time I open Mail on my computer. You can even use filters to automatically sort into folders if you wish.
I don't know how it is on other computers and/or with other programs but, with Apple Mail, each email is saved as a file. They're hard to find and they tend to be sorted many layers deep into folders, but they're there and you can access them directly. If you sort into your own folders using the application, the individual files are easier to find.
You can save these files on backup disks, either manually or with backup software (Note: some backup software excludes mail files by default but most will allow you to mark them for backup).
Tip: After you've been using it for a week or so and are sure you have everything, set the application to automatically delete messages on Sonic's server after X amount of time (I use one month). This avoids the problem where you find yourself about to be charged for being over your disk space allotment and have to scramble to fix it before the deadline (it takes a couple steps to delete messages quickly as moving them to the trash will not instantly change your disk usage; you have to empty the trash too).