Yahoo error 999 means that Yahoo has temporarily suspended your account/IP due to too much activity. You should disable the account in POP Peeper for 24 hours before trying to access it again; try once every 24 hours if the problem continues. If you have received this message, then you need to reduce the amount of traffic that you use from Yahoo. Here are some suggestions:

• Reduce how often POP Peeper checks the account; the minimum recommended interval is 15 minutes. You can specify the interval for an individual account by following these steps:
  1. Select "Set Options" under the main "Options" menu
  2. On the "General" page, check "Enable Advanced Options"
  3. Press OK
  4. Edit the yahoo account (right-click on the account and select "Edit")
  5. Select the "Advanced" tab
  6. Change the polling interval
  7. Press "Update"
• Don't retrieve the entire message, set it to headers or preview. Headers is recommended if you receive some messages that you won't read (e.g. spam). You can change this for the account on the account's "Advanced" page (see above for instructions on accessing this page)
• Retrieve only the first page of the inbox (Options / Message Retrieval). If you do retrieve multiple pages, then you should keep the number of messages in your inbox to a minimum by deleting messages and/or moving them to another folder. Set the number of messages per pages to the maximum (200) which will greatly decrease the bandwidth (and time it takes to retrieve the headers).
• Don't retrieve the body for messages marked as read on the server (Options / Message Retrieval)
• Set POP Peeper to delete messages at the next mail check, which is the default option. To check and/or change this setting, go to the "General" page in Options and verify that "Delete Mail Immediately" is unchecked.