I installed the Virtualmin Mail Relay 1.0 plugin and followed the instruction here http://www.virtualmin.com/documentation/id,mail_relaying_with_virtualmin/, but when I try to configure the mail relay for an existing server I get "Relaying is not set up for this domain".
What do I need to do to get this to work and have the form appear so I can configure the remote mx server?
Sounds like maybe you skipped a step? (I’m not sure, as I’ve never used this plugin!)
If you have done everything in the guide, I’d suggest filing a ticket. It’s either a bug in the docs or a bug in the plugin. 
when I looked at the postfix configuration the plugin did configure it to relay mail for the domain. I will redo the installation and configuration process to make sure something did not go wrong or was skipped.
I just reinstalled and reconfigured and still the same problem. I am going to open a ticket.