Sharing an SMTP E-mail Domain between Exchange Organizations

First, the official word from Microsoft on how to do this. They will explain in more detail. See the following KB article and associated linked articles:

http://support.microsoft.com/kb/321721/

For example, if ABC Company (ABC.COM) and XYZ Company (XYZ.COM) want to both use the SMTP domain called ABCXYZ.COM then this is how you do it:

My example is between two Exchange organizations. Only one Exchange organization can be authoritative for the new domain. The authoritative Exchange organization will send a NDR back for any unresolved addresses. Which Exchange organization you pick to be authoritative depends on many factors such as:

- which organization will hold the majority of objects using e-mail addresses in the shared SMTP domain
- one organization is taking over another

Let’s assume ABC Company will be authoritative and the MX record for new SMTP domain has been configured.

On the ABC Company’s Exchange Server, add in a recipient policy to accept mail for ABCXYZ.COM. Make sure you check the box stating “This organization is responsible for all mail delivery to this address”. On the recipient policy, you can choose to add a filter so that new objects that qualify automatically get an e-mail address in ABCXYZ.COM address space.

Before proceeding, you should verify mail flow in and out of your Exchange environment using e-mail addresses in the shared SMTP address space.

It might be worthwhile to set up a direct connection for mail flow between the two organizations. Otherwise, mail flow will go out over the Internet and you may not be able to control the routing of messages. This can be done with an SMTP Connector. You will probably need to deal with firewall rules, IP addresses of bridgehead servers on both sides. The SMTP connector on ABC Company’s Exchange will have an address space of *.XYZ.COM. A similar SMTP connector on XYZ Company’s Exchange will have an address space of *.ABC.COM.

Now we are ready to add the second Exchange organization to the mix. Since ABC Company’s Exchange is authoritative for ABCXYZ.COM, it needs to know about EVERY e-mail address in ABCXYZ.COM — even those in the other Exchange organization. This is done with the use of mail-enabled contacts in ABC Company’s Active Directory. The contact will have two e-mail addresses - anyuser@XYZ.COM and anyuser@ABCXYZ.COM. Make whatever e-mail address you want to be the primary SMTP address. In order for mail for anyuser@ABCXYZ.COM to reach the other mailbox in the other organization, you need to modify the target address of the contact. The target address is the e-mail address showing up on the Exchange General tab of object. Just make sure you still have two e-mail addresses under E-mail Addresses tab - using ADSIEdit to modify the target address is the easiest way. Think of the target address as a forwarding address for contact - i.e. where you want the mail to go - i.e. to anyuser@XYZ.COM mailbox on XYZ Company’s Exchange. Don’t forget to add anyuser@ABCXYZ.COM as an e-mail address on mailbox on XYZ Company’s Exchange!! Now, any mail addressed to anyuser@ABCXYZ.COM will get re-directed to anyuser@XYZ.COM over ABC Company’s dedicated SMTP Connector.

To control mail routing for ABCXYZ.COM from XYZ Company’s Exchange, you should set up an SMTP Connector with an address space of *.ABCXYZ.COM on XYZ Company’s Exchange. Also, the recipient policy for ABCXYZ.COM on XYZ Company’s Exchange should not be checked as authoritative.

As a side note, you will need to have some discussion between the organizations on the distribution of e-mail addresses in the new address space. For example, if ABC Company uses the e-mail address sales@ABCXYZ.COM and XYZ Company uses the same e-mail address. ABC Company will get all the messages addressed to sales@ABCXYZ.COM from the Internet since it is authoritative.

That’s about it. To see how to do this with a non-Exchange mail system, check out the Microsoft article.