Click Tools, then Accounts
Click on your POP/SMTP account that needs to be changed and click Properties.
Clear the checkbox that says "Leave a copy of messages on the server"
Do not change any other settings on this window.
Hit OK, and keep doing so until your settings are saved.
Next time you download your mail, it will clear out the backlog on the ideaLEVER Mail server.
It WILL NOT delete any messages from your mail store on your computer.