Kemical, you might want to hide your e-mail a little better...
I never liked the way the Mail App handled my pop accounts. Unless I was doing something wrong, it seemed I had to select each account to see what mail was present for that account. So I was never sure if the listings were up to date.
But it should be checking for new mail in all the accounts and that is one of the Apps, I believe, that you can allow to run in the background.
Since I don't have Gmail I can't really check..