[CentOS] Mail server setup for small ISP

Hi,

	I will probably have to design an e-mail (and other components) 
infrastructure for a small ISP soon (WISP).

	I'm doing some research to determine which components would be best to 
offer e-mail services to their client and allow the staff to manage 
accounts easily.

	I usually use virtual machines a lot for isolation and easy backups and 
migration (when a hardware node is underpowered, it is easy to migrate 
one or more virtual machines to another hardware node easily).

	I have looked at iSCSI and drbd for high-availability of the storage: 
http://www.pcpro.co.uk/realworld/82284/san-on-the-cheap/page1.html.

This looks like it should be doing a great job of high availability storage.

	For mail server, I guess I should look at an MTA and IMAP/POP server 
that supports LDAP and/or MySQL for users.  Postfix should be a good 
choice for MTA, as I know it (at least a little, but I know sendmail 
better).  For IMAP/POP, I'm not sure...  Would dovecot be sufficient, or 
should I try cyrus.  I'd rather use components that are available for 
base or extras repository (or rpmforge).  I think that squirrelmail and 
horde would do a good job for webmail.

	There shoudn't be any troubles having some redundancy for DNS, web 
servers, mtas, but what about IMAP/POP? linux-HA?  MySQL replication 
should be enough, I guess.  Or maybe linux-HA as well.  I wonder if I 
should add GFS to the mix to have multiple IMAP/POP servers use the same 
storage.  Or maybe IMAP proxies?

	Any insights welcome :).

Ugo