@ajk21,
Are your attachments being pruned? I don't know Yahoo mail well, it's been years since I ever used it.
Incidentally, Windows Live SkyDrive might be a better option for you. I haven't used it but they say they give you 25GB of free online storage. It might be a better free solution for backups, but if you really want to do backups right I recommend paying for the storage through Amazon S3, and using a tool like jungledisk.