Exchange 2003 Multiple Smart Hosts
You probably already know how to set your Exchange 2003 server to forward all outgoing mail to a Smart Host (another smtp server). But what if that server is down – you would want a secondary server to use as a backup.
If you want a secondary SmartHost for the same connector, the format is:
In other words, simply list both Smart Hosts in the box, surrounded with square brackets and separated with a comma (no spaces).
More information: http://support.microsoft.com/kb/265293