After you add a second or successive clustered mailbox server (CMS) of a Microsoft Exchange Server 2007 Single Copy Cluster (SCC), you cannot create a mailbox on that CMS. You receive the following error in Exchange Management Console:

Warning If you use the ADSI Edit snap-in, the LDP utility, or any other LDAP version 3 client, and you incorrectly modify the attributes of Active Directory objects, you can cause serious problems. These problems may require you to reinstall Microsoft Windows 2000 Server, Microsoft Windows Server 2003, Microsoft Exchange 2000 Server, Microsoft Exchange Server 2003, or both Windows and Exchange. Microsoft cannot guarantee that problems that occur if you incorrectly modify Active Directory object attributes can be solved. Modify these attributes at your own risk.

To enable mailbox creation on the second or successive CMS of an Exchange 2007 SCC, you must manually create the message transfer agent (MTA) object in the Active Directory directory service. To do this, follow these steps:

1.	Click Start, click Run, type adsiedit.msc, and then click OK. Note The ADSI Edit tool is included with the Windows Support Tools. To install the Microsoft Windows Server 2003 Support Tools, double-click SUPTOOLS.MSI in the Support\Tools folder on the Windows Server 2003 CD.
2.	In the ADSI Edit Microsoft Management Console (MMC) snap-in, expand Configuration [domainControllerName.example.com], expand CN=Configuration,DC=example,DC=com, expand CN=Services, expand CN=Microsoft Exchange, expand CN=GUID, expand CN=Administrative Groups, expand CN=Exchange Administrative Group, and then expand CN=Servers.
3.	Right-click CN=CMS_ServerName, point to New, and then click Object. Note Replace CMS_ServerName with the name of the clustered mailbox server for which you want to enable mailbox creation.
4.	In the Create Object dialog box, click mTA in the Select a class list, and then click Next.
5.	In the Value box, type Microsoft MTA, and then click Next.
6.	In the Value box, type 20 to set the transTimeoutMins attribute value to 20 minutes, and then click Next.
7.	In the Value box, type 5 to set the transRetryMins attribute value to 5 minutes, and then click Next.
8.	In the Value box, type the netbios name of the CMS object under which you are creating the MTA, set it to the mTALocalDesig attribute value, and then click Next.
9.	Click Finish to create the MTA object, right-click the new MTA object, and then click Properties.
10.	In the CN=Microsoft MTA Properties dialog box, click the Attribute Editor tab, and then click to select the Show optional attributes check box.
11.	In the Attributes list, click legacyExchangeDN, and then click Edit.
12.	In the Value box, type a legacy distinguished name (DN) that resembles the following: /o=organization_name/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=CMS_ServerName/cn=Microsoft MTA For example, type the following in the Value box: /o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=Server-1/cn=Microsoft MTA
13.	Click OK two times, and then exit the ADSI Edit tool.