Proposed simple redundent distributed mail network

Problem: A completely redundent mail system is required where the possibility of loss of mail is minimal, and accessibility of mail is high. The system must be easily producable, with very low start-up cost.

Background: During a week of power outages caused by floods, many domains on my single server lost access to email. No mail was lost, but the lack of availability of the mail was of concern.
It occurred to me that a simple and distributed mail network could be developed using existing protocols and technologies.

Design: All servers have the same directory structure for the storage of user mail, and the same authentication data for all local users.
This flow chart illustrates the life cycle of new incoming mail:
Each mail server in the network is in the MX record for the domain, and also runs an IMAP daemon to allow clients to check messages.

Benefits:

• Inbound mail is propagated throughout the network regardless of which server accepts the message
• Regardless of which server the user connects to, their entire mail store is available to them
• In the event that a server become unavailable, new mail will automatically be delivered to another available server, queued there for copy-delivery to the offline server. Users will also be able to che available servers for their newest mail.

Concerns:

• When a servers marks a message as "read" on one server, this information also needs to be propagated to the other servers
• Likewise, when messages are moved between folders or deleted
• When a server goes offline, clients may receive "Conneciton Refused" or "Connection Timed Out" errors.