When the cost of the Send Connectors and the proximity to their source servers are the same, Exchange will simply choose the one with the alphanumerically lower connector name, and will not load balance the outgoing email across both connections.

The correct solution is to deploy a single Send Connector with multiple smart hosts.

Read more at All Spammed Up…

Related posts:

• How to Share an Email Domain Between Two Mail Systems
• The Case of the Hub Transport Server Load Imbalance
• Report Top Sender IP’s on Exchange Server 2010 using Log Parser
• TechEd Video: Load Balancing with Microsoft Exchange Server 2010
• Avoiding Infinite Loops with Internal Relay Domains in Exchange 2007/2010

This article How to Correctly Use Multiple Smart Hosts to Load Balance Outbound Email for Exchange 2010 is © 2010 ExchangeServerPro.com

Get more Exchange Server tips at ExchangeServerPro.com

• Two companies merge but maintain separate systems for a period of time
• Non-Exchange systems are involved in the email environment, eg a Unix Sendmail server

Because Exchange Server handles this so easily it is generally best to send email to that domain name to the Exchange server first, and then let Exchange determine which recipients are local and which need to be sent on to another host.

First take a look at the domain name that you want to share.  It is likely configured as an Authoritative domain in your Accepted Domains list.

[PS] C:\>Get-AcceptedDomain | fl domainname, *type*

DomainName : mycompany.local
DomainType : Authoritative

To share the namespace we simply change the DomainType to Internal Relay.

[PS] C:\>Set-AcceptedDomain mycompany.local -DomainType InternalRelay

Now that the domain type has been changed we need a Send Connector to tell Exchange where to send the emails that do not have a local recipient.

[PS] C:\>New-SendConnector -Name "Internal Relay" -Custom -AddressSpaces mycompany.local -SmartHosts 10.8.0.25 -SourceTransportServers ex2007.mycompany.local

Identity       AddressSpaces            Enabled
--------       -------------            -------
Internal Relay {smtp:mycompany.local;1} True

In that example the IP address of the other email system is 10.8.0.25, and the Hub Transport server to use to send the emails is ex2007.mycompany.local.

Now when an email arrives on the Exchange server that has no matching local recipient address, it will forward it on to the other mail system for delivery.

Update: In the comments below and via email some readers have described situations in which this does not work as simply as I outline above.  The most common scenario reported to me is that it doesn’t work without setting up Contacts in each organization for the users in the other organization.  Without those Contacts emails to recipients in the other organization result in an NDR.

This can happen when recipient filtering is occuring on the Exchange servers, or on a server running a third party anti-spam product, that rejects email that is addressed to invalid recipients.  This check is usually performed with an LDAP lookup against Active Directory.  However in a shared SMTP namespace scenario like this, any given Active Directory does not contain all of the valid recipients for all of the organizations sharing the namespace.

So to avoid NDRs when using shared SMTP namespace you will need to either disable recipient filtering, configure the product to do LDAP queries against all directories that share the namespace, or create Contacts.

Related posts:

• Avoiding Infinite Loops with Internal Relay Domains in Exchange 2007/2010
• How to Migrate a Relay Connector from Exchange Server 2007 to 2010
• Review of CodeTwo Exchange Rules Pro
• Exchange 2007/2010 Transport Rule Logging
• How to Correctly Use Multiple Smart Hosts to Load Balance Outbound Email for Exchange 2010

This article How to Share an Email Domain Between Two Mail Systems is © 2010 ExchangeServerPro.com

Get more Exchange Server tips at ExchangeServerPro.com

Configure the Receive Connector

To allow the Exchange server to accept incoming email from the internet the default Receive Connector must be modified.  Navigate to Server Configuration/Hub Transport.  Open the properties of the default Receive Connector.

Select the Permission Groups tab and enable the Anonymous Users group.  Click OK when complete.

Configure the Send Connector

Navigate to Organization Configuration/Hub Transport.  In the Actions pane to the right of the Exchange Management Console click New Send Connector.

Enter a meaningful name such as Internet Email and set the intended use to Internet.

Click Next to continue.

Click the Add button and add an SMTP address space of * to route all mail to external domains over this Send Connector.

Click OK and then Next to continue.

If you route your outgoing mail via an ISP smart host or email security service choose that option and enter the IP address or DNS name of the smart host.  You can add more than one smart host if necessary.  Otherwise leave it configured to use DNS to route mail directly to the destination.

Click Next to continue.  The Hub Transport server is automatically included as a source server for the Send Connector.  Click Next to continue, then New to create the Send Connector with the chosen settings.  When the Send Connector has been created successfully click Finish.

Allow the Exchange Server 2007 server to send email to the internet

Add a rule on your network’s firewall to permit the Exchange Server 2007 server to send traffic to the internet on TCP port 25.  On an ISA Server 2006 firewall the process is as follows.

Open the ISA Server Management console and navigate to <ISA server name>/Firewall Policy.

Click on Create Access Rule in the Tasks pane on the right side of the ISA Server Management Console.

Give the new Access Rule a meaningful name such as “Permit Outbound SMTP”.  Click Next to continue.

Set the Rule Action to Allow.  Click Next to continue.

Leave the Protocols set to “Selected protocols”.  Click the Add button and choose SMTP from the Common Protocols list.  Click Add again to add SMTP to the list of permitted protocols for this Access Rule.

Click Close to close the Add Protocols selection dialog, then click Next to continue.

For the Access Rule Sources click the Add button and then click New -> Computer.

Enter the name and IP address of the Exchange Server 2007 server then click OK.

In the Add Network Entities dialog navigate to Computers and select the computer object you just created.  Click Add to add it to the new Access Rule, then click Close.

Now that the Exchange server is showing in the list of Access Rule Sources click Next to continue.

In the Access Rule Destinations dialog click Add, navigate to Networks select External then click Add and Close.  Click Next to continue.

Leave the User Sets configured to All Users.  Click Next to continue, then click Finish to close the New Access Rule Wizard.

Apply the ISA rule changes.

Related posts:

• Publish incoming SMTP to the Exchange Server 2007 server with ISA Server 2006
• How to Share an Email Domain Between Two Mail Systems
• Route outbound email through the Exchange Server 2007 Hub Transport server
• Publish Exchange Server 2007 OWA Using ISA Server 2006
• Generate SMTP Error Statistics using Log Parser and Exchange Server 2010 Protocol Logs

This article Configuring the Exchange Server 2007 Hub Transport Server is © 2009 ExchangeServerPro.com

Get more Exchange Server tips at ExchangeServerPro.com