Enterprise qmail-ldap operations, scalability

[Forrest Aldrich]

I'm doing some research to facilitate building an enterprise-level email 
system, for a VISP-type environment, that would potentiall support 100's 
of thousands of users (maybe more).

Each user would have at least 5 mailboxes.  And for this application, I 
believe a directory service would come into play.

A few people I've spoken with have expressed concerns about OpenLDAP 
performace under such high load, opting for MySQL or something else.

I wanted to quantify this and inquire to others out there who may have 
more experience, perhaps some advice/tips or anecdotes of your own 
experience in this scenario.

Thanks in advance...