The problem with the mail settings, the problem is the configuration of the plugin, the problems with mail on the server and so on....
I suggest You:
1) to start is to enable debug in WP and see the logs.
2) to See the logs osiak PHP.
This is one of the main rules. If something is not working it is most likely something or don't like WP or PHP.
Alternatively, you can install the plugin WP-Mail-SMTP. It has the ability to send messages and error logs (you can understand the problem with the mailer or not).