Sorry to hear of the troubles, but if it's any help, in 13 years I never heard of this with Macs (I hope you're not the One!). You can probably figure the problems are system software related if OSX Mail is being affected.

If you still have the free AppleCare, get them to walk you through the usual troubleshooting steps (safe boot, repair permissions).

If you don't have AppleCare, http://www.macattorney.com/tutorial.html is pretty good for general troubleshooting on Jaguar, and you can always search the Apple discussion forums for the error message and see how others have dealt with it. If you post, they'll need specific versions of iBook and system.

oops--I see you already cross-posted there. Good luck.