I've had a couple of the groups I'm backing up start downloading messages later than the earliest. In one case, the earliest it would start downloading was 16891 - messages from February 2003 - but the group began in April 2002… (A LOT of messages to not be backed up!) I can see them listed in the calendar, but they're not available to be seen, much less backed up. So if anyone else encounters this, it's possible that the earliest messages cannot be accessed anywhere at all for whatever reason, and thereby will not be able to be backed up unless Yahoo figures out what's going on with them (which I highly doubt they care about doing).