Postie processing emails immediately, to on specified interval

When I send a new email, Postie processes it almost immediately. If I send two or three emails, a few minutes apart, they all are processes immediately. This, regardless than I have specified twice per hour.

The problem with this behavior is the Postie appears to be constantly checking and causing a load on my server which in turn causes my ISP to throttle my site.

http://ww.wp.xz.cn/extend/plugins/postie/