confirm delivery/read BB to BB only? network dependent?

I've spent an hour reading every thread on confirm/read and still can't figure this out. BTW, it seems everyone is also confused--sometimes it works, sometimes it doesn't...

First, does this only work if you have a @blackberry email address? I have a Gmail account linked to my Blackberry. I have checked YES to Confirm Deliver, Confirm Read, Send Delivery Receipts, Send Read Receipts.

When I send mail from my BB to someone, I don't get any confirmations. If someone (Gmail) sends me an email, they don't see anything or I don't see anything either.

Also...I've read that not all carriers support this function. I'm on AT&T--so maybe AT&T doesn't support it?