Hi and thank you for contacting our support.
Newsletter uses WordPress to send emails (there is a famous wp_mail() function everyone can use) and WordPress relies on the hosting provider, if not configured to use a third-party system.
If the hosting provider is not good at sending emails or has imposed limits, filters, and so on, the email could not reach the subscriber, without notice.
With a delivery add-on active, Newsletter “talks” directly to the email delivery service.
See our email service integrations on https://www.thenewsletterplugin.com/documentation/addons/delivery-addons/
Regards,
Roberto