IIS 6 Smart Host Outbound Security

I'm using the IIS 6 smart host setting to forward all email sent through IIS to SendGrid.

It works fine if I set the Outbound Security and smart host settings at the top level of the Default SMTP Virtual Server properties.

But the downside with that is if I want to switch it off or change the smart host, I have to also edit the Outbound Security at the same time.

I thought I could get around this by creating a domain smtp.sendgrid.net and setting the Outbound Security on that while leaving the smart host setting at the top level.

My understanding was when it tried to forward through the smart host it would pick up the Outbound Security settings from the specific domains properties.

So my question is, does this just not work or am I forgetting some extra step to get this working?

I saw some reference to having to restart the SMTP service before it would pick up settings on the domain, but I tried that to no avail.