Organization of LDAP

["Administrator" <lists@digitalplace.org>]

I am going to be hosting multiple domains on my server and will be offering
email addresses to these companies.  I am trying to organize my LDAP
database before building it and am unsure of the best way to do it.
Originally I thought about doing:

                                                        US
                                                    /            \
                                                domaina    domainb
                                        /            \
/                \
                                Accounts    People        Accounts    People

The problem that I see with this is I don't know how / if you can tell the
IMAP to authenticate against multiple trees?  If I can do that will this
organization layout work.  If I can't do that can anyone suggest how to use
ldap to manage multiple domains?
Any help would be greatly apprecaited.

Thank you in advance for your time.

Michael