OS 10.4 - Mail.app - @mac.com + @gmail.com

I have recently purchased a .Mac subscription, and i would like to make the switch of my main email address to my new @mac.com. i want to use my existing gmail account as a second backup location of all my mail (both @gmail.com and @mac.com), as it is free and i will still be using using my gmail account.

i have attempted to set this up, but have crossed a few wires in the process. now when i test send (from a crappy hotmail account) an email to the @mac.com account, it also sends a copy to the gmail inbox (both web interface and mail.app. i know this is logical if i have set it up to send a copy to gmail for storage, but i have no forwarding on.

is this possible, or do i have to recieve in both inboxes to archive all mail in my gmail account?

thanks in advance

rhys