Where I work (government) we're searching for a replacement for an 
email system.  The current system has custom hardware and software 
and runs on (would you believe) a private X.25 WAN.  It gets messages 
to any or all 50-odd sites throughout the Western US in less than a 
minute.

Here's the new game plan/requirements:
1. Solution needs to be "off the shelf" h/w and s/w.
2. A private and robust IP WAN is in place, ready for use.  The 
Internet doesn't guarantee delivery times.
3. A pair of servers that are data mirrors should be at one node.
4. Another server mirroring data can be located at another node on 
the WAN.
5. Loading is very light (something like 10 to 20 1k messages per 
client per hour).
6. Reliability is very important, re: the robust WAN and multiple 
servers.
7. Failing from one of the paired servers to the other should be 
nearly unnoticable to the client users.  Failing to the second-site 
server could require stopping the client and relogging in.
8. Copies of messages sent from any client go to the intended 
client(s) and to a repository (could be one of the servers).
9. A convenient means of determining whither a given message has been 
delivered.  A return msg from each destination is not a good solution 
as 50 such messages are just clutter, and one or two missed 
deliveries would not be noticed, but should alert an administrator.
10. Email client must have a forms capability.  That is about 10 pre-
defined forms exist on servers and clients.  User fills in the 
blanks, and only the variable data is transmitted.
11. Logins (user, client maintenance, server maint., & sys 
management) must use encrypted passwords.
12. Automatic printing of received messages at client sites.
13. A folders capability for manual and/or automatic sorting and 
retention of messages at the client.

Yes, it's a tough spec.  M$ Exchange 2000 comes somewhere near 
fulfilling the requirements, but at something like $5k to $10k 
licensing per server!  If there's something close in the Linux 
environment, I'd like to take a real hard look.

Gene