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.
SBS Server - Small Business Server 2003 where you email is directly delivered tobackup mail server - Another SBS server or Exchange server where email will get delivered to if your SBS Server is offlineMX - Mail Exchange recordYourDomain.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 email@example.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 -
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
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.