[ale] Recommendations for mail server w/ LDAP?

[Mike Harrison]

On Sun, 2 Dec 2012, James Taylor wrote:
> Maybe I'm missing something, but 20gb mailboxes and hundreds of thousands of messages in a single folder usually don't scale well on a flat file mail system.
> I work with that scale on most of the systems I support (not exchange, very, very fortunately..) and they need a regularly indexed database to keep them from falling over at best.
>
> Historically, ISP scale systems consisted of a lot of email boxes without huge amounts of mail on the server in each folder.
> Of course the current biggies are more like what was described above. But I doubt any of them use a flat file mail system.

Hence my disclaimer. Pre-GMAIL, few people stored their main mail archive 
on the mail server.. Then, POP was the more popular standard with most 
people downloading their mail and deleting it from the mail server on download.

His question, and one I am also interested in the answer of,
is what works better? What is current best practice for that kind of load?

What FOSS or FOSS-like available software supports that best practice
at less than gmail/yahoo.com scale?