HI!

So, at the end of a long journey:

An issue like this invokes if UNA hosted on VPS servers. By default, sending mail is not available on all plans. But you need to set up an RDNS (reverse DNS) by hosting panel. In my case, I have set my domain as RDNS value. Now verification email received on a few tested email addresses. I hope my investigation helps to somebody.

Best Regards,

Alex