Email Address Problems

We experienced a new issue yesterday and need your help. Fast Secure Form would not send a form to a particular email address, but it would send to other emails. As a result, we had to deactivate this plugin and use a different plugin. I need to know why certain emails worked and certain other emails did not work. Also, if a email address did not work, what happened to any forms that may have been sent? Are they lost?

https://wordpress.org/plugins/si-contact-form/