Sharing SMTP address spaces in Exchange 2003

Mike mailed me with the question of the day… 

 If the “This Exchange organization is responsible for all mail delivery to this address” option is not selected on an Exchange recipient policy then will Exchange forward mails to a smart host if it cannot find an Exchange mailbox associated with an email address or contact?

So I had a look at the KB articles on Setting up SMTP domains for inbound and relay and about sharing SMTP address spacesExchange treats e-mail messages differently depending on whether Exchange is authoritative or non-authoritative for a particular SMTP address space. To view this setting:

1. Open the properties on a recipient policy, click the Email Addresses tab, click an SMTP address, and then click Edit. 
2. Examine the setting for the This Exchange Organization is responsible for all mail delivery to this address check box. If this check box is selected, Exchange is authoritative for the SMTP domain. If this check box is not selected, Exchange is non-authoritative. 
Exchange must always be authoritative for the primary SMTP address (the one in bold) on the default recipient policy. Otherwise, local mail flow may not occur. When you share selected SMTP address spaces, a shared address space is set as non-authoritative in Exchange. If the address space that you want to share is not the primary address on the default recipient policy, click to clear the This Exchange Organization is responsible for all mail delivery to this address check box for that domain.

If you share all Address spaces then Exchange becomes authoritative for all address spaces… here’s what the kb says:

This configuration is much simpler, but much less flexible. In this configuration, Exchange is authoritative for all address spaces. You cannot have any contacts in your directory that have a target address that matches a domain that Exchange is authoritative for. To apply this method:

1. Open the properties on the default SMTP virtual server.
2. Click the Messages tab.
3. In the Forward all messages with unresolved recipients to host box, type the FQDN or the IP address, in square brackets ([]), of the server that will receive unresolved mail.
4. Click OK.
5. Repeat this procedure for the default SMTP virtual server on all Exchange servers except for any virtual server that is acting as an inbound gateway for the other system. Microsoft recommends that no mailboxes reside on this server.
Note that this setting only affects authoritative domains. Therefore, for any authoritative domains, messages that are sent to unresolved addresses are forwarded to the server that is specified on the SMTP virtual server. Any domains that are non-authoritative in Exchange 2000 or Exchange 2003 are not affected by this setting. Messages that are sent to unresolved addresses in non-authoritative domains are routed to a matching SMTP connector if a matching SMTP connector is present. If no matching SMTP connector is found, the message is sent to the server that is specified in the MX record that is found in DNS.

So its a simple matter of ticking the box… but make sure you know what domains you’re authoritative for – or you won’t get mail!