[bogomipz]

Comcast is a "last mile" ISP or "eye-ball network." Technically they are tier 3 ISP> They buy transit from Tier 1 ISPs. Tier 1 ISPs "peer" with all the other Tier 1 ISPs.

So you might be a Comcast and they might be transit from Level 3. Your friend might be a customer of some small ISP who buys their transit from AT&T.

Level 3 and AT&T are both Tier 1 ISPs. All tier ISPs have an agreement that they will route their respective customer's traffic to from each other's networks. This called peering. hey have peering agreements with each other. More specifically Tier 1 ISP do what is known as "settlement free" peering with each other because no money changes hands. And these are not legal contracts they are just hand shake deals, its an exclusive club basically. There are only a handful of tier 1's although it is a regional distinction. Tier 1 ISP in the US are not necessarily the same Tier 1 ISPs in Australia for instance.

Comcast only has an incentive int that they would have angry customers if those customer couldn't send emails to their friends or family members who use another ISP. This being said there are occasional peering disputes where the internet becomes partitioned and customers of 1 ISP can not reach customers and site on another ISP. And customers do get upset.