[CentOS] Mail server setup for small ISP

>        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 :).

www.directadmin.com

Been running it on CentOS for years.  Added Clamav and spamassassin to
it.  It utilizes Exim and dovecot along with standard bind and apache
stuff.  You pay monthly or yearly license fee.  Its pretty cheap
really.  You can also pay a one time fee for a given machine.

Matt