I have (or had…) the same issue. After long scrutinising that dialog box I finally realised that there was an extra button “add email address” that actually was the submit button.
What I did: fill the first part of address (a domain was preselected) and suddenly there it was… took me a while to discover that it was a submit button.
OK, I might understand, although not convinced, about not displaying the submit until som conditions are fulfilled. But would it too much to ask to make it then visible aka distinct from other buttons (see screengrab below)? The “cancel” button might deserve a distinct look as well.
While I am at it: that focus ring (around the domain name <select />
, pointed by arrow) is barely visible. Maybe try to give it sufficient contrast with the background? Accessibility 101…