Scaling IMAP and POP3
Another scalability strategy brought to you by Erik Osterman:
Just thought I'd drop a brief suggestion to anyone building a large mail system. Our solution for scaling mail pickup was to develop a sharded architecture whereby accounts are spread across a cluster of servers, each with imap/pop3 capability. Then we use a cluster of reverse proxies (Perdition) speaking to the backend imap/pop3 servers .
The benefit of this approach is you can use simply use round-robin or HA load balancing on the perdition servers that end users connect to (e.g. admins can easily move accounts around on the backend storage servers without affecting end users). Perdition manages routing users to the appropriate backend servers and has MySQL support.
What we also liked about this approach was that it had no dependency on a distributed or networked file system, so less chance of corruption or data consistency issues. When an individual server reaches capacity, we just off load users to a less used server. If any server goes offline, it only affects the fraction of users assigned to that server.
