This article is for SBS users who want to have a backup mail server just in case you SBS server has an outage (power, internet troubles, hardware, etc).  Most ISPs will offer some backup mail solution for a fee, but if you have another exchange site or maybe another business with SBS you can do this for free, if both businesses are willing.

Terminology:

SBS Server - Small Business Server 2003 where you email is directly delivered to
backup mail server - Another SBS server or Exchange server where email will get delivered to if your SBS Server is offline
MX - Mail Exchange record
YourDomain.com - fill this in with whatever email domain you own

First step is to get MX records added -  You can request to your domain name registrar or whoever hosts your DNS records to add a second MX record with a lower priority.  Your primary MX record will point to your SBS server with a priority of 0 or 10.  Add a second MX record that will point to the backup mail server's hostname or IP with a priority of 30.  If this is setup properly mail going to bill@yourdomain.com will try to contact the SBS exchange server first.  If it can't contact the SBS exchange server then it will try the backup mail server with the lower priority.  Thus ensuring that mail will get delivered to the SBS server if it's up and running.

Second step is to configure the backup mail server to accept mail for yourdomain.com and hold it until the SBS server request the mail - 

• From Server Management (SBS), expand Advanced Management, YourDomain(Exchange), right click on Connectors, select new -> SMTP Connector...
• Name the connector something
• Add a local bridgehead, select the local exchange server's name
• Under the Delivery Options tab, select Queue mail for remote triggered delivery (no account is added for ETRN)
• Under the Address Space tab, add an SMTP address of YourDomain.com, cost 1
• Check Allow messages to be relayed to these domains

Mail to YourDomain.com will now get queued up when delivered to this backup mail server.

Third step is to configure your SBS Server to download the queued mail - This step assumes that you are getting your mail directly to your SBS server

• From Server Management (SBS), expand Advanced Management, YourDomain(Exchange), Connectors, right click on SmallBusiness SMTP connector, select properties
• Under the Advanced tab, select "Request ETRN/TURN from different server"
• Enter the host name or IP address of the backup mail server (use host name is your backup mail server is using a dynamic IP)
• Specify a schedule of how often you want to check for mail on the backup mail server (I do every 4 hours since I don't anticipate my SBS server being down very often)
• Select "Issue ETRN", click Domains... button, add the email domain that you want delivered.

You should now be setup to download queued mail from the backup mail server.  Most of the time there will be no mail to download since your SBS server will be up and running, thus receiving all the email to YourDomain.com.  In the case of any long outages to your SBS server, you're email won't be lost.