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