@bostondave hey David, you can achieve that but with a custom sign-up form.
Use a text input element and a button.
On the button, add an action to sign the user up.
But set some conditions: the text input must contain @thedomainyouwant.com.
And on the button, set a visibility condition to only appear if the text input contains @thedomainyouwant.com.
If you want the “not allowed” message to appear, you might want to have that underneath the button in the editor. So if the user inputs email@example.com, the button will appear and cover the “now allowed” message.
There’s no “does not contain” condition at the moment, so this is as close a workaround as there is.
Hope this makes sense