I want to configure SMTP on my web server, so that any email sent through the SMTP server is relayed to a remote SMTP Server. The IIS SMTP server would have to use SMTP authenti
I think you can only set outbound relays for specific domains, not blanket coverage.
http://www.isaserver.org/articles/smtprelayinboundoutbound.html
EDIT: I've not done this before, buy maybe worth a try:
From the Server properties, you could try selelcting the 'Delivery' Tab, then advanced. In the Smart Host, type the outgoing SMTP relay IP / domain. Select OK, and then select 'Outboud Security' and enter your username / password in the basic authentication box.