All emails are bouncing
Please confirm SMTP details are correct. Please ask hosting provider "What are the SMTP details for my PHP script?". Please try changing MAIL_SMTP from 'true' to 'false' in the config.php file as well, or try disabling SMTP in the system settings page if that is an option.
Please try sending a "preview" to your own email address to see if that produces any particular errors.
Send through FTP details and Newsletter login details as a support ticket if you are having trouble and we can investigate why emails are bouncing.