Paul Vixie has been talking about mail peering for years. Unlike him I personally do not believe exact equivalent of BGP peering for mail is a solution and will ever happen. But there is intermediate altenative - create organization with all isps as its members (kind of like ARIN/APNIC/RIPE for mail service providers) and have all downstream corporate customers be required to either also be member of this organization or relay email through its isp. Do note that right now already many new isp customers relay mail as well as all dialups, but actually making this work for number of large corporate customers is a problem but if we really want this to happen, we can!
Perhaps we should move mail transfer to a peering model. You wanna send email to my SMTP server? Where's the peering contract? BGP-equivalent for SMTP anyone?
-C (tired of getting bounces for email I never sent!)