Hello,
I just found out about this site. I normally use YahooPOPS but I will check this out and see how it works.
I just got a 999 error which may be a result of some limit. So far I have seen people talk about number of messages sent as well as time spent as variables Yahoo might use to determine when an account should be locked up with a 999.
May I also suggest it might keep track of what intervals an account is accessed. For example, if you make it wait 1 hour before downloading more messages, Yahoo might have a limit as to how many times it gets mail every hour. If it retrieves mail in this fashion, then 24 times with an hour apart could possibly be a factor to block an account.
I say this cause I don't come close to 20-30 emails a day. I also regularly delete messages. I don't spend much time, but I do have email programs check on a regular basis. If people chose an hour interval, perhaps there could be something added like randomly pick 1-10 minutes to jog the times it checks email so it isn't always an exact hour. Then Yahoo would have a harder time to determine if it is automatic. There is also the possibility they have a range set up.