Dear Martin,

One think which you should consider after starting your IIS service is granting the permission of relay e-mails through your SMTP server.

1- Open IIS
2- Right click on Default SMTP virtual server and select "properties"
3- click on "Access" tab
4- At the end of the opened page click on "Relay..." button
5- Select "All except the list below" radio button and check that there is no entery in the list below of the radio button
6- Click on the Check box below the ADD button to make it checked, the check box is following with this title "Allow all computers which successfuly autenticated ..."

Hope you run it.
Farhad