I'm sorry you're having trouble sending yourself a test email.
Are you working on a draft or a sent email when trying to send yourself a preview?
Are you receiving a error message when trying to delete your old email address and put a comma to try to put somone else's email address in?
What browser are you using? If you are using Internet Explorer 11, we are aware of a issue, could you try another browser?
If you can answer these troubleshooting questions, I can take a deeper dive and try to figure out why this is happening