Sharing a domain between two Exchange orgs

Luke asks:

I am in the process of migrating from Exchange 5.5 to Exchange 2003 for a company that has multiple small (less than 5 users) branch offices.

They want to migrate slowly, so here's my dilemma: The current Exchange 5.5 will still be active for a while. I'm using the Exchange Migration Wizard in Exchange 2003 to migrate the mailbox data over.

All incoming mail needs to be split between the two servers, as the new Exchange 2003 server will be gradually hosting more & more of the users.

Can you point me in the right direction where I can learn to write an SMTP event sink to route some recipients to one server, & others to the other? Thanks!

I can't join the Exchange 2003 server to the same Exchange 5.5 site, as it's in a different domain.

Luke, first a question and an assumption.  When you say that the Exchange 2003 and Exchange 5.5 servers are in different domains, do you really mean that they're in different AD Forests?  That you haven't done anything to join the two together, that you just want to start fresh with a new AD that just has the Exchange 2003 server in it?  I'm assuming so.  There's no need to write any code to do what you want to do.  What you want to do is exactly the same as sharing an SMTP domain between two totally separate Exchange orgs.  What you need to do is to make one the “hub” for the domain, where all inbound mail comes in to, and it forwards other mail to the other system.  And good news, there's a KB article that tells you how to do it:

XADM: How to Set Up Centralized SMTP Domain Sharing in Exchange 2000 Server for Independent Organizations

Comments (5)

  1. mike says:

    Unfortunately this KB isn’t totally accurate. Step 6 states: "…Because the main SMTP-address of the Windows 2000 contacts is, recipients in the Organizations B through D appear outbound with this SMTP address." This doesn’t appear to be true.

    Mail from "Organization B" routes through Org A, but the from address still ends up as for internet recipients. Very bad if either does not have a valid MX record on the internet or you need to have a unified e-mail address presented to the public.

  2. Luke Edson says:


    Wow, thanks for such a speedy answer, I had looked at that MSQ article, but let me explain further:

    There are two domains, A, & B. They have a two way trust between them. – The current Exchange 5.5 server is receiving all mail for, the Exchange 2003 server is receiving all mail for – I am collapsing domainA into domainB; that means the recipients of domainA will end up with two SMTP addresses, &

    The clients will be migrated over from domain A to B over time, so there will be a period where the Exchange 2003 server will be receiving mail for domainA only for a couple of recipients, & needs to forward all the rest of the recipients (for domainA) to the Exchange 5.5 server.

    What I’m looking for is some way of the SMTP service to say, "oops, has to get forwarded to the Exchange 5.5 server" (They are both accepting & sending emails, but only the Exchange 2003 server has the external MX record pointing to it.)

    In other words, the Exchange 2003 server is hosting the mailboxes of & The Exchange 5.5 server is hosting the mailboxes of & Since they’re not in the same domains, I can’t set them up to be able to talk to each other to let each other know which one is hosting a specific mailbox.

  3. Luke Edson says:

    Thanks David,

    I had a mental block as to what my problem was, & how to fix it, but after reading the MSQ 315511 article more in depth, & trying it out, that’s solved the problem!

    Thanks again.

  4. Ralf says:

    The more wider used KB-Article for SMTP Address Sharing is;en-us;321721 (mainly method 1)

    and Mike is correct – if the sub-Org does not have as the primary SMTP Address for its recipients – then either Address rewriting like for Exchange 2003 in or You will have to setup MX records for the subdomains

  5. mike says:

    Hmmm, didn’t realize that rewrite tool existed, thanks for the link. I’m assuming it won’t work on Exchange 2000.

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