Multiple Organizations send and receive email as one

Do you try to keep old addreses or are you trying to kill them yesterday?

The old addresses will be kept for sometime to maintain reply to functionality on old mail, but moving forward, all email sent and delivered will be from a single namespace.

I can propose exim or postfix mailer which can integrate in 3 ADs and serve as temporary mail router for old addresses afterwards.
You can also keep old addresses in exchange as long as you have internet domains at hand.

There is a proposed migration to a single AD/Exchange Org, but that is later down the road

That is just not possible.  It has to be now because that is the way it is done.

Also important!  There is no such thing as an AD/Exchange Org,...the Domain and the Exchange Org are enirely two separate and unrelated things.  Yes Exchange relies on AD,..but that has nothing to do with the Exchange Org.  You can create, add, remove Exchange Orgs to your hearts content and the AD Domain is never touched, nor does it care.

You must create a new Exchange in a New Organiztion,...or pick and exisitng one that you want to keep.  Then you can uninstall and reinstall Exchange on the other Exchange boxes and join them into that particular Exchange Organization as you re-install them

It is not clear to me if you also have multiple Domains,...if you do then you need to migrate the users into the Domain you are going to keep (or create a new one?).  When the users are migrated AD will create the new mailboxes for the Users on the Exchange that is a member within that Domain.  

You can then Export the Mailboxes from the previous Exchanges to PST files using Exmerge as long as the mailboxes are under 2gig in size (they should never be allowed to be anywhere near that large in the first place).

Once the Users and Workstations are in their permainent Domain Home using their new empty mailboxes on their new Exchange,...then use Exmerge to import their old saved mail from those PST files into their new mailboxes on their new Exchange.

What you do wtih the Mail Domains is up to you.  Exchange allows multiple addresses per user and also will work with multiple Mail Domains (note I did not say AD Domain),...it's just a matter of configuring the Recpient Policy correctly and adjusting the Public MX Record on your Authoritative DNS.Hosting Provider.

When all that is done and working correctly you can take the servers from the other Exchange,...remove Exchange,...join the machines to the correct Domain,...re-install Exchange ahd direct it to the correct Exchange Org as you install it.  Then you would simply use the Move Mailbox feature to move the mailboxes to whatever Exchange you wanted them to "live on".

thanks pwindell,

let me clarify:
3 Seperate companies involved in merger
(3) AD Forests - Each company has its own AD and Exchange Org therefore
(3) Exchange Orgs-Each company/forest has an Exchange Org

Migration would indeel be the path of least resistence, but we do need an iterim solution as migration is not yet an option. Business drivers require that all the above entities be able to send mail as and receive mail using a single smtp address (@company.com).

Migration of users/compueters/servers/mailboxes is a long term goal and will require funding, planning, busness acceptance...etc. As I mentioned, the need is to use a single external smtp identity prior to and eventually during a migration coexistence.

Thanks again!

Then you have exactly what I thought.

There is not going to be any "interim".  You have to to the full migration from beginning to end as quickly as possible,...you have to "git 'er done".

If you cannot do that now then you have no option but to just leave things as they are until you can do the migration.  Trying to do anything other than that is sure to be a disaster and even if you had a measure of success it would only make the final migration more difficult,...certainly not easier.  This is definately not the first time this has been asked or tried.

It is possible to choose the final Domain (or create the final Domain) and also the final Exchange Org that goes along with it and then do the migration while the old Domains and Exchange Orgs remain intact and functioning.  Remember that the Migration does not damage or destroy the "old" in the process excep the workstation migrations which you cannont do till the final moment.

Once ready then you adjust the MX Records and then you just have the users begin using the new Domain by Migrating their machine and do the Security Translation.  Then delete/recreate their Outlook account profile and transfer their mail with Exmerge,...and you are pretty much done.

Note,...since these are 3 different companies it may be impossible to run them in the same domain and have the same Exchange Org.   Multiple Exchange cannot interact fully together unless they are in the same Org,...and they cannot be in the same Org unless they are in the same Forest (if I am not mistaken).  So you may have to go with a Multi-Domain Forest or a Master/Child Domain model in the same Forest.

So there is a lot to consider here and there is just no way I can answer those things for you.  You are the only one who can do that,...and if you cannot then you may need to bring in outside help from a Consultant who can,...at a minmum,...help draw up a plan to work from.

Time spent in research while awaiting comments from the group led to a solution for my issue.