Syncplify.me Server! is a secure file transfer server, not a mail server. Therefore, if you want your Syncplify.me Server! to be able to send out emails (for example a notification when a user tries to log in with the wrong password) you will have to specify the SMTP settings. Basically this is the host name or IP address of a mail server that is authorized to send out email on your behalf, along with all the necessary authentication information.
To find out the correct configuration for your SMTP server, please refer to your ISP's documentation.