I helped design and setup 2 large SMTP infrastructures for two different European mobile carriers. Both carriers had more than 1 million customers and processe

I helped design and setup 2 large SMTP infrastructures for two different European mobile carriers. Both carriers had more than 1 million customers and processed close to 100 million messages a day. It was about 7 years ago but I doubt things have changed that much. We made heavy use of Mirapoint. But we also made use in different places of qmail and postfix. At the high end of scaling SMTP routing starts becoming similar to routing IP traffic. You're just not going to beat a dedicated device like Mirapoint with some Unix box you make from scratch. You need dedicated and differentiated mail routers for inbound, outbound and mail storage. You monitor their usage and when they get overloaded you just add more. Since everything is load balanced you can scale close to linearly. We used SAN for storage. Local storage just doesn't work. Mail hits an inbound SMTP router and that router does an LDAP lookup to find which storage box actually handles hat account. Then it forwards it. That's all it does and it does it fast. Each storage box handles mail for the accounts it stores and IMAP/POP3 access for those accounts as well. The outbound routers just take mail from the storage routers and spool it for outbound. I could go on but I think you get the idea. The main idea is to put everything behind a load balancer that you can so that you can scale it linearly as much as possible.