Instructions for setting up gmail
I would suggest you use an external redirect service which adjusts your host mx records to allow them to filter the mail before you retrieve it. Essentially they act as your mail server.
This one charges $0.25 per address and redirects your emails to their servers, filters them and delivers them to your inbox ready for download.
Gmail will eventually reduce the traffic but in the meantime the server load will not decrease. The host will naturally reduce your bandwidth in order that other users of their service will not be affected.