Use this option to setup SMTP for all outgoing emails from your installation. This is useful if you are finding the WP-Client email notifications sent from your domain are being filtered out as spam by your Clients/Members. Use the “Test email” box to enter a working email address, and click “Test” to send a test email to that address using your entered SMTP settings.

The exact details will vary depending on who you would like to use for SMTP. Here is an example article on setting up Gmail as an SMTP server:

Following that article, the WP-Client settings would like something like this:

Just click "Test" to send a test email to the secondary email address.