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Distributed mailsystem

i take an interest on large instalation of mailsystem based on Zmailer with high scalability option (generaly hundreds of thousands active users). 
I would like to get some battle-proven recommendations about Zmailers cluster architecture. I have some ideas:
1.) one shared storage mainly for common $MAILDIR/ subsystem with 2 or more frontend servers running:
- Zmailer                ... SMTP delivery, (local delivery ?)
- CYRUS-IMAP (?)  ... local delivery, IMAP, POP3 access (while secure design, having non-system based accounts, support for user quotas and many more features - thats just idea, it could be possible that there are better choices)
It could be possible to run an L4-aware switch in the front of the system, providing virtual services for clients and distributting load between physical machines. Some machines could be propably dedicated for SMTP delivery and some for POP3/IMAP access to common message store or to run concurently all services on all of cluster nodes. Question is, how Zmailer/CYRUS works with shared message store, concurently accessed by many servers (DLM - dynamic lock manager feature ???)
(Webmail access could be an independent cluster based on one of freely available webmail solutions  - access to message store via IMAP/POP3)
2.) central/distributted message store (database) server + local delivery agent/modul running on each Zmailers frontend, POP3/IMAP-aware servers for accessing message store via XML/LDAP/... I dont know any freely available server with analogous model.
Thanks for any useful informations and contributions about problem,
Martin Pala