It looks like if you're running a virtual SMTP server in IIS7, the only GUI to configure it properly is in the IIS6 management console, in the "SMTP virtual server" section. The IIS7 management console "SMTP E-mail" icon gives virtually no configurability in comparison.
Isn't this a bit strange? Why didn't Microsoft port over the SMTP GUI to IIS7 instead of telling you to use the old IIS6 interface?
To answer my own question, the reason for this is that Microsoft (rather quietly and stingily in my opinion) deprecated the IIS SMTP server, although they didn't fully remove it. For this reason, you can select and install "SMTP server" feature in Windows, but you have to use the IIS6 management console to manage it because MS didn't bother porting over the GUI to IIS7 or IIS8.
I always wondered why IIS7 gave you 2 management consoles - the new one and the old IIS 6.0 console - and the sole reason is to be able to configure SMTP.
This is covered more in the following question:
What replaces IIS SMTP server in Windows Server 2012